Well, the major difference between Young Ryze and Championship Riven is that Championship Riven was available to purchase in the in-game store at one point, so bringing it back to the store makes it available in the exact same way it was before.
Young Ryze on the other hand has never been available in the store. It's a special Collector's Edition skin given to those who pre-ordered the game. In a way, it's kind of like an "achievement" skin in that you did something "special" and you got the skin, and the time frame for that "something special" has long passed. I know that's not too great of a comparison, but it's the closest I could think of. Making it available in the store would bring it back like other Limited Edition skins, yes, but it would be available through a different method than it was before.
I'm not saying that there's no chance Riot will bring it back, just that because it was never available in the store to begin with, the chance they do bring it back is very small. That's just my take on it anyway.
As for going through the trouble to update it and everything, they pretty much have to do that for the OG owners of the skin. Even if they don't have any plans to make it available again, the players that do have the skin might still be playing, and Riot needs to make sure the skin doesn't get left behind for those players.