In my Biased opinion, This is a way for Riot to generate profit from OTP and dedicated Mains, as they have a hard cap on Skin sales for that kind of player. I don't personally like the look of it, I am willing to wait and see the full implementation and will reserve judgment until that happens.
I believe its possibly viable as a positive, but to do so requires the gains from the Eternal system to be consistently viewed as worth the $. If I was a Red, I would ensure that on launch there is a "Eternal" Skin line ready to go for each and every one of the games champions. One that tied to the Eternal but clearly represented the champions thematic as well. That's a massive undertaking on a scale never done prior, but if properly implemented the potential Gains over several Years would pay for it.
Otherwise it needs to just be low cost so that its just an additional way to express in game dedication and pride in your achievements.
Anything else is shit IMO and not worth supporting. Gut reaction for me is negative but I am willing to wait to see full implementation.
Disclaimer, I happen to like Riot and have been playing and supporting the game from season 1.