Skins don't feel rare enough to me
This is just a personal opinion but I believe it has some depth.
When I see somebody with an epic legacy skin, I just sit there and think, "wow, I guess he got that during the time the vault opened" It just does not impress me and makes collecting seem kinda useless.
An example would be like on CS:GO. There are like skins that were only available for like 3 months and it makes it super cool when you see someone with one. I don't feel like there is enough of that in league. I like collecting skins for rarity but if the vault is just going to open up every year, then that makes my legacy skin not feel as special because people can buy it straight out and I had to open chests to find mine.
It's just a concept and I know that gemstones have helped with this and also very rare skins like rusty blitzcrank, king rammus, and ufo corki. Those 3 skins are the only ones I have ever seen people freak out about.
TL;DR Legacy skins need to be more rare so people can show them off.