I have mixed feelings about this. On one hand, we really do need a huge IP Sink and I like the idea of selling certain skins for MASSIVE amounts of IP (50K and up). While this would certainly be a high-five to active and long-time players, it would also create a whole new market for botting games for IP. Think about it, out of every 10 people who play this game probably 2 or 3 of them would be willing to pay someone to bot IP for their account to get these skins illegitimately. So you'd probably have a quarter of the people who own these IP skins be people who actually earned them, and three fourths would probably just be bots and botters who want the skins so they LOOK like they've been playing a long time.
Also, from a financial viewpoint there's simply zero profit in this for Riot. Now now, don't insult them for that, it's true. Companies need money to operate. It's not greedy at this point, it's a fact. Now, charging 1350 RP per skin is a bit greedy IMO but that's another topic entirely.
If they were to implement an IP sink regarding skins I think it would be as follows:
50,000 - 80,000 IP for a random skin out of a predetermined pool of skins. Basically, an IP self mystery gift, but with restrictions.
Restrictions:
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1 - 3 per year maximum, or whatever the average amount of IP earned by an active player could afford. This is to discourage botting, as you wouldn't be able to get more skins than the above-average person anyway. It's not flawless, but it sets a fair cap.
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The pool of skins would be limited to skins that do not sell very well, and maybe some of the Legacies that still don't sell even when they leave the vault. Basically, you wouldn't see any "popular" skins in them. Think of it as a bargain bin. This minimizes profit loss on Riots side (if you get the skin for free you won't buy it with RP if you otherwise would have), while it also maximizes advertisement of the skins (Someone sees you playing with a generally unpopular skin but they think it's neat so they go out and buy it).
Keep in mind that the price of this IP skin chest would NOT be tuned for everybody to be able to afford whatever the cap is every year, but rather as a reward for those of us who play a LOT. So potentially everyone could afford one, but the more actively you play LoL the more frequently you can purchase them.
Also, having any "IP chest specific" skins would just encourage botters a whole lot more than it would encourage active players, so I think that would be a bad idea.