[devonin]

So if songs have a relative numerical difficulty, and based on your scores on those songs, you have a level which is used to compare your skill to others, your whole issue is in saying "Let's put this particular range of those people together into a group"?

I mean, in a perfect world where we had 1 million active players, wouldn't it make sense, for example, to have all players of one single skill level compete against each other? That's literally divisions, just 110 of them.

their placement in an arbitrarily defined ever-shifting group of players

Every system of assigning difficulty/level/division/etc to anybody is arbitrarily defined. And the lines are only "ever-shifting" if we keep deciding to shift them around.