attracking and keeping more players

I suppose this is NOT going to be popular with really high level players ... the diamonds, platinum and above ... but then, how many players are in that group? 5%? 2% ...

The majority of players are going to be average ... by definition, half the players are below the median. If the distribution of player ability follows all other human abilities, then 68% will be "average" with 95% being with slightly wider abilities. There will be totally bad (lower 2.5%) and totally good (upper 2.5%).

How many are in those groups that are beyond the normal player you meet? if there were just 1 million players, that would be 25,000 players that are so intrinsically good that Joe Average will never beat them.

But sometimes I feel like riot puts all effort into the 25,000 at the top, and ignores the middle.

PSSST! riot! that is a recipe for disaster. The vast majority of your customers are going to be average players. (Us jokers that actually spend money on skins and stuff). If you make it so difficult for Joe Average to enjoy the game, Joe Average will play for a while, then uninstall. You lose him, you lose money. Not sure if it is enough money to justify your existence, or if it is a minor revenue stream, but when you lose a player, you probably don't get them back. So how do you make it better for Joe Average? Make it much more likely that he can succeed in game. If he plays, no matter who he plays, he should have a reasonable chance of winning.

How you want to accomplish that is up to you. It is your business after all. But remember who pays your paycheck (hint: it isn't riot, it is the customers.)

Easier for the 68% in the middle. Remember that if you remember nothing else.