Forbidding Google from requiring exclusivity is not the same thing as mandating that Apple accept payments from others.

Google can afford to pay more per user/click because of scale economies; their cost per user/click is lower. So, great, Google will pay Apple $20/user/year on a nonexclusive basis, and Firefox or whoever are free to match or exceed that, so long as they don't mind losing money on every user.

So the problem is that Firefox does not find its users to be as valuable as Google's?