I'll stick my hand in, as much as my gut tells me not to, because well, screw it. On the question at hand: No one. Everyone could have developed if they kept the League. It's not like anyone disagrees with that. In the dev blog, Riot said they could do it, but essentially didn't want to. Why? Because the stories involved were, in their opinion highly contrived and sometimes, not all of the time, things being contrived kinda sucks. Here's my thing here: this question seems like a trap because contrivance solves it completely. An example:
Vel'Koz's 
is the super cool lasery void floaty guy of doom. His purpose is to consume and learn, but his ultimate goal is to attain the ultimate knowledge. He believes Zilean 
, floaty timey whimey old guy of Urtistan who he's never met holds this knowledge. This means he wants to meet and kill Zilean, because Vel'Koz learns by lasering things to oblivion.
So Vel'Koz goes around, gets some intel, finds Zilean, and then stops because there's an institution of people in his way. At this point, the story goes one of two ways:
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Vel'Koz says fuck these sods and tries to fight his way to Zilean, and winds up imprisoned.
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Vel'Koz decides to be sneaky about it, either by hatching a plan to trick the summoners or covertly getting through their defenses to get to Zilean.
The first option kills his story dead in its tracks and puts him in the "Next step of the story is his break out!" a story that Cho'Gath, Brand, Renekton and Nocturne all suffered from. These summoners can keep a primal fire elemental, a monstorous void creature, a (former) alien warrior and a living nightmare imprisoned potentially indefinetly. So what happens then? A contrivance to solve the problem. A rogue summoner breaks him free, he somehow manages to figure out how to break free despite the sheer implausibility of such a thing, malzahar breaks him out, which is also pretty implausible considering that the Summoner's are probably sensible enough to not let him do that. Either these contrivances (there's likely more I haven't thought of) or some big institute shattering event would need to happen.
The first part of of the second option is unlike Vel'Koz. He's not the kind of entity to negotiate well with humans, especially considering that his kind are not so well received in Runeterra. The second option is more plausible but this is the INSTITUTE OF WAR. They'd figure it out. If they didn't it'd be poor writing IMO. If the institute can figure out how to keep so many baddies in check, what about Vel'Koz is so sodding special that he can't be? How is this solved? A contrived story to sort it out. I actually can't figure one, and kinda don't want to, because I feel like it'd be insulting Vel'Koz to do so, and I really like the little guy- wait no he's huge. I really like the big guy!
A contrived story can work, and be compelling, but you can't do it too many times or else it just feels really cheap. This is why filler anime is generally always bad, at least in my opinion, because in order to make it a reasonable threat, it has to have some kind of powers and involve certain characters in ways that mess with things. The contrivance there sucks, and is wears you down really quickly.
In League's case the same applies. If one imprisoned character escapes, the rest are fucked. Because logically, the rest of them would never be able to escape because the methods for keeping them imprisoned would drastically improve, and by extension their chances of escaping dramatically fall to around 0%. If they wanna make those characters get out and develop properly, they have to compound things further and make an even BIGGER contrivance to allow for it. For me, that shit would get old really fast. You can only pull contrivances once or twice before they get old.
So in the end I have to ask, not who couldn't develop because of the League, but who couldn't develop without a contrived story to push them along? To be clear, contrived stories can be great, but League would need to many.
Why does this guy exist if there is no institute of war? He's just a mercenary who was good at fighting in the league and wasn't known for anything before that.
