The issue is that it's hard to define when "being a veteran" starts at (unless you go to the absolute first few accounts that were created).
The farther back you go, less and less people get the reward that Rioters are spending their time on (versus something else such as a VGU or a skin that everyone can purchase) and more people are left upset that they aren't being included in whatever reward.
People are still upset about Black Alistar, Silver Kayle, King Rammus, etc. Chances are Riot isn't too eager to make more rewards that exclude the majority of their playerbase (especially seeing how previous iterations of obtaining Prestige Skins / content (green chromas, Sona emote) locked behind the Partner Program were received poorly by large portions of the community).
Just my personal thoughts of course (I'm a longtime player myself so chances are I would get the rewards) but I imagine most people would be upset, and it just seems unlikely (but not impossible) for Riot to prioritize something like that over something everyone could enjoy.