I agree that (in an ideal world) cosmetic items should be the only purchasable IAP. However, I sincerely doubt that cosmetic items alone would be justified revenue for the time put into making and sustaining EoE.
I also agree with Nez that I don't think playing against a stacked (purchased) deck would be as fun.
If the item buying power (through keys) remains the primary IAP - a few things have to be addressed (which I'm sure devs are aware of).
1. Level item requirement - We are told items will have level requirements. This is IMO the best factor in reducing the overpowered IAP ability. Obviously the balance has to scale and still be worth it to give some advantage to the spender. BUT who's to say someone won't drop 100$ every 5 levels to have the best possible PvP gear. Which brings me to-
2. Skill vs Items - Again, the balance needs to somehow favor both if keys -> chests-> gear . People will need to see enough improvement in their abilities with the purchased gear to continue doing so, but skill HAS to be an equally important factor. Hearthstone was used as an example of this balance.
3. Ability to earn similar items - Whether it's time, persistence, grinding, ect. Time grinding for gear as opposed to questing or PvP is still valuable gaming time. Again, the balance is delicate and paying for keys has to have some kind of incentive.
TL;DR version of this post-
Devs need the game to make money and it's understandable that they need to find a balance between IAP and earned content rewards. I don't blame the devs for doing so, but I understand the frustration at IAP when gear is greater than (or equal to )skill. Cosmetic stuff is the safe way to go , but I doubt it gives the devs the return they really want.