While I appreciate the idea to give a reward for reaching mastery 7 on a champion, the particular idea to give us a chroma for the base skin is not really doing it for me....
It would still be alot of work, we are almost at 150 champions at this point and each of them would have to get a chroma for this system. A chroma they would hand out for free, too. And to top this all off.... I don't see that many people actually using such a chroma.
I have many champions on mastery 7, among them every single one from Piltover and Zaun. And I have skins for almost all of these Piltovians and Zaunites, too. Quite a bunch of the base skins are just outdated, so I highly prefer to play these champions with a skin, which makes them look more clean and fancy. I wouldn't pick the old baseline skin in my game just to show off a mastery 7 chroma.
Unfortunately, it is really hard to come up with a good reward for the mastery.