I think that you have it a little off, it's not that league isn't competitive anymore. It's almost exclusively competitive even, just look at riot's behaviors and the kind of people that still play the game, don't you dare try to play something that doesn't have the competitive advantage, else face the wrath of every player in your game, 3 players from next game, the announcer, and fucking baron (probably).
The term "casual" has never managed to settle on a definition, but I think it's fair to say that you can see an audience that is best catered to by the game's current design. Unfortunately it's too small of a demographic and most players are bleed over from one of the other two target demographics the game has historically had. The committed audience and the Noncommittal audience.
Committed: Learned every champion, have internal timers on base CDs for every champion, knows skills/details about the game that have since been added to the game's UI (e.g. buff/dragon spawn timers). These players aren't necessarily "competitive" playing ranked mode, but they do enjoy the game enough to want to learn as much as they can about it.
Noncommittal: Has a hand full of champions that they know well enough to win matches, have a good amount of experience in their role/class but don't know the first thing about, likely, supports (except for maybe Pyke), wants quick games, these are the kinds of players who will frequently use 3rd party software to pick their build, and really just care about the excitement in a match rather than the details about the game.
Sadly a lot of the noncommittal players (as the name implies) are quick to jump over to other genres that offer more of what they're looking for, but especially the ones who aren't familiar with PC FPS games aren't likely to make that transition to the current action trend of BRs.
But, I just realized I'm rambling again, in an attempt to make a game that catered to both these committed and noncommittal players, riot ended up with a game that best fits players who are noncommittal and competitive... Players who don't care to learn a ton about the game outside of what will get them those sweet sweet W's and as few L's as possible. This specific category of players isn't very large, because it's nearly impossible to make a cohesive game that only requires very narrow knowledge about the game. Yet (according to a lot of people who play the game) the ranked system tries to enforce a 50% win rate anyway, so good luck with that
[sg-lulu]