There could be an argument that allowing alternative stores would just mean every app would switch to those and be able to work around Apple's rules and act against consumers' best interests.

However, the fact that Apple only enforces the rules when it benefits them as opposed to consumers (so they are happy to turn a blind eye to spyware, but will crack down immediately on alternative payment options) means that in practice I don't see it making the situation any worse.

>There could be an argument that allowing alternative stores would just mean every app would switch to those

Which is a ridiculous argument as the network effects and default preference status of the App Store mean it'll still be pretty much a requirement to publish there if you want any users.

Not really.

Apple currently has the market power to keep Facebook and other similar scum at bay if they chose to (unfortunately, they're not doing anywhere near enough on this front).

Opening the floodgates to alternate stores would simply mean every mainstream service out there would require you to use that alternate store (isolated from Apple's enforcement) where all kinds of nastiness is permitted.