The game is becoming far too complicated in terms of both design and player skill to not have a focused group of champions.
There are 137 champions in LoL and so many items, runes, masteries and variables to account for.
For example, if I wanted to play Vlad to his fullest.
https://gfycat.com/gifs/detail/LeftDifficultEyra
I'd have to be able to pull off difficult maneuvers like this. Playing a champion to an ideal level takes hundreds of games on them. If I played even just 100 games on 137 champions that is 13,700 games. More than most people have even played LoL. To be sufficiently good at all the champions. Even then with your 100 games on Darius you will still get your ass handed to you by somebody with 900 games on Riven or Rumble.
Ideally the average LoL player should have one main champion, a couple backups and a moderate pool of champions they have decent skill on. For example.
BoxBox:
Riven he plays about 60% of his games
He has Gnar, Katarina, Lee, Fiora, Ezreal and Gragas he can play to decent levels and take up about 30% of his games.
The other 10% is a mix of a slightly more broad pool of champions, just in case he needs them or grows an interest in them.
The point isn't that I am incapable of playing Vi. It is that I would perform far better on Hecarim, Shyvana or Zac. It is unrealistic, arrogant and foolish that you think anybody, yourself included, can play all, or even a large portion of the champion pool to an equal and sufficient level for the tier of play that they play in.