You should get a prestige skin for a champion when you reach 1 million mastery points with it. it does mean that Riot needs to make a prestige edition skin for every champion, but I think it would make more sense.
Except that Prestige skins are not supposed to be rewards. They're status pieces for those who want an extra way to show their support of the game. That's the entire point of them. It's why they are released alongside a regular version of the skin, so that if you really want the fantasy of Fizz as a dog or Kai'Sa as a k-pop star, you have that option.
