I don't think the difficulty of the champion should directly be proportionate to their power. For example, let's look at Annie. Annie is a champion unlocked extremely early and is hella easy to play. Despite that, she's a top pick in pro games. I think the more difficult champions to play aren't exactly "stronger". Easier champions don't have to be "weaker" either. I admit that Riot hasn't exactly had the best balancing, but with a game like League, making the best champions the more difficult ones to play is kind of detrimental to the whole design. I love the fact that some of the easier champions can still be played in a top tier environment (like Ryze). Besides, I think the mechanics, knowledge, and skills that you acquire the longer you play are more than enough rather than "if I play enough to get x champ then I will acquire all the elo."
I don't think I understand very well. You say that "making the best ones the more difficult ones to play" is in someway a detriment to the whole design, but in what way? On what basis do you make this claim? In what way would it be detrimental to game design? What you're saying is that I'm wrong, because of annie, essentially. There's no facts here.
Please explain to me how rewarding more skilled and more experienced players is a detriment to the game, considering this is a practice Riot already implements in this game.
For example when you first join the game you're at a tremendous disadvantage, because you don't know the ins and outs very well. By playing a lot you start learning things like timing, other champions abilities and what those champions are weak against. You're consistently learning a plethora of different skills and knowledge which helps you become a better player, until it becomes more like a sense than anything else. This creates a sense of natural rewards as you keep playing.
The game is already balanced around skill inherently, if this game lived in your dream world where easy champions where powerful, it wouldn't be taken seriously on a competitive level.
Because rewarding players for doing things the easy way is fucking stupid.
I don't know what to say about annie, I've literally never seen her used in a pro game. She's weaker than both Ziggs and Vel'koz, both champions requiring more skill than her.
If you look at the champion Kha'Zix, he's an intense, super fast champion that has great potential. But if you're not quick enough he's not very good. Basically he rewards skill with power, assuming you're skilled enough to utilize said power.
A character being easy to play is it's own reward. If you want an easy champ you should miss out on the potential a champion like Kha'Zix has.
So you could say I disagree with you. Making champions very easy AND powerful is what is detrimental to game play design, not just in league but in any video game.

This is in no way precise, because I made it real quick in ms paint so yeah.
It's just a guide line, it's not like I expect everything to adhere perfectly to it.
It's the fact that Master Yi is WAY off the curve of the skill/power ratio.
Meaning he's too easy, or too powerful or both.