I support the idea of a single store for security purposes, but that's not Apple or Google's goal. If it was then 30% of apps on the Play store wouldn't be malware and Apple wouldn't be fleecing its developers so hard. What's the solution? Add an easy-to-find setting that allows people to use any store they like while agreeing to have their device assigned to an isolated segment of the manufacture's online infrastructure.
This is already the case with Samsung. I have used their store to install a few things, and I also use AuroraDroid to install some apps from F-Droid. They suck. They don't have nearly the same selection and integration as Google Play Store.
Personally, what I would want to allow for Play Store/App Store extensions. Something like apt's PPA and dnf's COPR system. The Play Store and App Store is too entrenched, and any additional stores that don't have the same selection, integration, and familiarity isn't going to matter, ever.
The best way to open it is to allow user and other store to plug into the Play/App Store UI. We already have account switching in those Store, so an integrated source switching is more than possible. Also, don't diminish any functions just because they have a different store -- people will just be too afraid to do it. Anything less will just be performative.
