For those new to the game, allow me to immediately disabuse you of a notion that you might have. Riot's record on reworking champions is incredibly spotty. One of two results usually occurs, they either correctly identify the problems with a champion and fix them, usually resulting an incredibly OP mess at the end of the day, or they fail to address any of a champion's actual issues and make a ton of irrelevant changes that often make the situation even worse. You don't need to look far to see this effect in action, as quite literally every assassin touched in the assassin class update either became ridiculously overpowered in the right hands (Katarina, LeBlanc) or would soon become overpowered once lethality was buffed. Even Fizz, who probably recieved the lightest touches of any of the champions looked at, had to be nerfed after a couple of patches due to being too powerful.
As you can probably tell from the title, Riot's recent pass at a Cho'Gath "rework" falls solidly into the second category. The issue with Cho'Gath is not that he's unable to scale into infinity with Feast, and giving him the ability to do that while ignoring the fact that he's utterly useless even if he gets that far is tantamount to a sick joke for anyone that actually likes the champion.
So what is wrong with Cho'Gath? The easiest way to see this is simply to compare him to the gold standard of his role and see where he falls short. A side-by-side comparison with Maokai will quickly reveal exactly why no one opts to take the largest insect in the void onto the Rift (unless they're looking to make a Riven sad...a job that Maokai does perfectly well anyways).
First, the passive. We're all familiar with Maokai's passive, Sap Magic. He sees 5 spells, he gets a ton of health back on his next attack, scaling with level. It's part of the reason why Maokai is so obnoxious to remove from lane, or kill in pitched battles where 20 spells can easily fly around at once. Cho'Gath's passive, Carnivore, has a similar function. Upon killing a unit, Cho gets some HP and Mana back. For the first 10 minutes of the game, this passive is one of the most powerful in the game, as it gives Cho'Gath lane sustain that other champions can only dream of. After 10 minutes, however, it's only marginally more useful than Swain's useless passive, as how many units is Cho'Gath really going to kill in a full five on five teamfight or even a smaller skirmish? Will getting back 25 health really make the difference when Dangerous Game is likely giving him 10 times that amount? As you will see "doesn't scale well into the later stages of the game" is going to be a rather recurring theme for the void cockroach.
Both Maokai and Cho'gath have a disruption spell as their first ability, and seeing how they compare and, more importantly, synergize with the kit of the champion that has them, tells us a lot about the reason why Maokai sees constant play while Cho has gone unpicked for a year. Maokai's Q is one of his most important spells, Arcane Smash. It does damage in a wide area around Maokai and disrupts the targets that it hits. It also sports a very short cooldown, a fact that makes it Maokai's bread and butter spells for all purposes. It also combos well with Twisted Advance, Maokai's root that puts him in range to actually cast any of the rest of his skills consistently. Cho'gath's Q is also his primary spell, but also his primary weakness. Rupture is an aoe knock up that deals respectable damage but comes with a -very- long delay. In theory, Rupture is one of the most powerful non-ultimate abilities in the game, as it can knock up and damage the entire enemy team. It is, however, a skillshot unlike the aforementioned Twisted Advance, and it's also Cho'gath's sole engage tool. In Magical Christmasland, where all of your opponents play on a trackpad with 300 ping, it's one of the most powerful engage tools in the game. In reality, it's a zoning tool that just screams "don't move here" to the entire enemy team. Not useless, but not much better than, say, Spirit Fire, which at least deals damage upfront. To make matters worse, Rupture also comes with a long, long cooldown, and only gains damage from being leveled up, damage you'll never see since it will almost never hit anything unless it's used as a follow-up CC -- a role it admittedly shines in.
Cho'gath's most powerful ability is, in reality, his W, Feral Scream. It's a nearly instant, long-duration, long-cooldown, and easily aimed cone that silences and damages everyone hit by it. For most mages, getting hit by Feral Scream and it's nearly 2-second silence is a death sentence, and it was rightfully toned down almost a season ago, as it made trading with Cho'gath in lane even more of a nightmare than it already was. Amusingly, though, even here Cho'gath falls short of his bark-clad nemesis, as Maokai's W is one of the most defining skills in League of Legends, Twisted Advance. Point. Click. Snare. Kill. That's what Twisted Advance does, and it's the primary reason why Maokai has remained an enduring force in League for such a long time, even when the metagame is unfavorable for him. It even deals percent damage, all while having a short cooldown! In short, Twisted Advance is everything a tank wants all wrapped up in a single button. Feral Scream, while admittedly powerful, is only a disruption tool, not an engage tool. If Cho'gath can't waddle his way into range of a relevant target, it doesn't do anything.
This brings us to the two champion's E, both of which are somewhat lackluster. Maokai's Sapling Toss, however, is still head and shoulders above Cho'gath's Vorpal Spikes. Sapling toss does exactly what it sounds like: Maokai throws a sapling at the target location that acts as a ward until it sees an enemy target, at which point it runs towards the targets and explodes, slowing and dealing damage. Vorpal Spikes add AoE magic damage to Cho'gath's auto attacks that does useful things like randomly drawing tower aggro, screwing up last hitting, and occasionally dealing a nice 15 damage to the enemy laner with no effort expended. For a tank -- a role that Riot obviously wants Cho'gath to inhabit considering their change to his ultimate -- it's hard to imagine a more useless skill, especially when the tank in question already struggles to get into range of any target that would actually care about the minimal damage his spikes do.
Finally, we ccompare the two champion's ultimate. For Cho'gath, this means examining his most iconic ability, Feast. Feast is a single-target true damage nuke that gives Cho'gath a stacking HP buff if it kills the target. It also does extreme damage to minions and netural monsters, more damage than even a max-level Smite can deal. Unfortunately, you can now only get five stacks of Feast from minions, and any above that must be earned by killing epic monsters or champions with the ability. For the purposes of this comparison, we'll assume that Cho'gath will have, on average, 6 stacks of Feast in any normal game, which grants him an additional 840 HP at level 16. Maokai's ultimate, Vengeful Maelstrom, is no less iconic, however. It puts up a vortex around Maokai that absorbs 20% of all damage he or his allies take while standing inside of it. Oftentimes, a single activation of Vengeful Maelstrom will prevent more damage than Cho'gath gains from his Feast, as the percentage damage reduction renders Maokai the single most difficult champion in the game to kill. Furthermore, that defense also extends to Maokai's team, and he can reactivate the ability to deal damage to all enemies within the field that scales with how much damage it's prevented. There's a reason this ability has seen multiple nerfs, it's one of the most dominant abilities in the game, especially if the enemy team relies heavily on burst damage.
The comparison here is stark. Literally every skill Maokai has at his disposal is as good or better than what Cho'gath has, which should rightfully bring to mind the question: why would you play Cho when you can instead play Maokai? The answer is simple. You wouldn't. It's telling that the last time we saw Cho'gath in competitive play it was specifically to counter a spell-spamming melee champion, and the time before that he was a mid laner. In both cases, he built AP, not tank items, making Riot's decision to push him in the direction of a tank even more bewildering -- he's literally never filled that role in a competitive capacity in the history of the game.
Thus, when players complain that Riot seems out of touch with their own game, there's a good reason behind it. It's unclear at this point whether Cho'gath even got more powerful from these changes, as the amount of health he can gain from killing minions is now locked at a lower number than it previously was, while none of his actual core issues were solved. The question Riot needs to ask themselves if they want Cho'gath to be a tank is this: what tools are you going to give him to start fights? When Maokai wants to engage, he pushes W and stuns the target for a good 2 seconds straight. If Nautilus wants to engage a fight, he hooks into range and uses his impossible to dodge disruption ultimate, Depth Charge, to knock up half the enemy team. When Malphite wants to start a fight, he hurls himself bodily into the enemy team with Unstoppable Force -- an ultimate that is, byt the way, roughly comparable to Rupture except it's faster and is still well known for being quite easy to dodge.
If Cho'gath wants to start a fight, he has to fire off Rupture, pray to Aurelion Sol that it actually hits something, and then waddle up to that target and use his only other two activated skills on them, then pray that's good enough since his cooldowns are so long the fight will likely be decided by the time it's over. If Riot wants Cho'gath to be a tank, they need to take a hard look at his kit and ask themselves how he's supposed to control or impact fights, because right now the answer is that he simply doesn't, and that makes him both a bad tank and a bad champion. Being able to stack up to 10,000 hp won't mean anything if the enemy Ashe just shoots you for free for thirty seconsd because your champion can't do anything to stop her.