Honestly people love to blame LCS Balancing, High Damage, and new Gimmicks, but

I think the actual culprit for gameplay dissatisfaction is way simpler: People are just better at the game.

League is really fucking hard. Incredibly difficult. The sheer number of things you have to be able to execute to play even at a mid-bronze level is insane. "But this game is so easy, and it's not Dota 2, and Bronzies are so bad..." No, let me stop you there. It feels easy, because you've put in a ton of time practicing it, and because you had some general knowledge from previous games. Imagine teaching your grandmother to play league. She doesn't know what cooldowns are, she doesn't know what mana is, she doesn't even know what HP is! And while any one of those things is simple enough to pick up, the sum total of all the things you have to keep track of in league is HUGE.

So what does this have to do with gameplay dissatisfaction? Well, as a game gets older, the playerbase as a whole tends to have played the game for longer. This 1. makes them forget all the stupid grindy bullshit they had to do to get where they are (See: any mmo where all the Veteran players complain about how the game is "Too easy" because the devs made it actually fun instead of a grindy snoozefest) and 2. makes them tend to be better at the game.

Things like Insec plays, keeping track of jungle camp respawns (thank god for timers), minion wave control, specific champion and item powerspikes, specific jungle pathing - these all used to be special tricks that pros and high level players developed to win more games. And then everyone saw them, and started slowly teaching themselves, and these high-level strategic and mechanical strategy became the default. Don't play Lee Sin if you can't Insec, don't jungle if you don't know the possible paths, don't lane top if you don't know when your enemy will hit level 2, and when the junglers will gank, and the lane priority and gameplay strategy for your matchup, and your botlane's matchup, and dragon timings, and tp timings, and your and your opponents and both junglers' powerspikes, and...

And knowing one of these things when your opponent doesn't, and using it to secure a lead, is deeply satisfying. But feeling like you have to know and keep track of all these things even to succeed on a baseline level, because your opponent does and if you don't you'll get dived with their jungler at level 3, starts to feel like a chore. It's the homework you have to do before you get to play the game, but it is the game. And when your ally isn't quite as on top of the current meta, and you try to rotate bot but they don't play around it and get their shit kicked in, you blame them. You get so mad at them, and it's other really their fault that they missed this detail, but it still feels like working on a group project with someone that failed to prepare.

This is why games feel so much more swingy and like a coinflip, why games feel like they're not yours to win but your allies to lose. Sure, team bounties and anti-snowballing mechanics contribute, but mostly it's because everyone is so much better. Your ally doesn't suck, he'd have kicked your shit in a year ago or two years ago... you're just that much better. And as everyone knows, as you get to a higher level, smaller and smaller differences look bigger and bigger. The best player in the world can make the 10th best look bronze, even though they're both incredible. Everyone's better, and so the difference between good and great looks like the difference between trash and good.