having multiple stores in itself is fine, but it's even worse worse. if you want to release your app on multiple app stores, you have to release on all of them at the same time. if you skip one they will not allow you to launch there later because you dared to treat them as second choice. imagine apple banning you because you dared to release on android first.
Because there's no point as they need their apps to be cross platform and so Apple is setting a baseline for what behaviour is allowed.
But once Meta, Epic etc can run their own stores on both iOS and Android they will absolutely move their apps to them. Epic has already said how important this is to be able to implement their metaverse strategy.
Epic already did this at the height of fortnite's popularity, by using a "fortnight installer" app that you had to get from outside the play store. Can't remember why they backtracked, or why they didn't offer store/sideload versions at the same time.
Google even took steps to prevent people from installing malware pretending to be fortnite, and steered them to the official installer. Not out of the goodness of their heart, TBF.
I really, really don't think it'll be as easy you as say. Neither the audience nor the regulators will let them off easy, and these creaking giants aren't exactly great at launching popular new platforms, these days:
it is happening for android - there is more malware in the third-party stores, even with the extremely minimal scrutiny google apples to the official store.
anyway, here is a real-world example of the type of spyware that facebook and others will deploy once they have the leverage. again, remember that this already happened and most likely still exists and is something they have ready to deploy again if the opportunity presents.
You don't have to come up with hypothetical scenarios if you already have a real life example to look at.