I could be wrong about this, but it seems like it would change for most users since Facebook/Google/Amazon/etc have a big incentive to push third party App Stores: ad tracking. Apple currently limits it, so I would not be surprised at all if suddenly all of grandmas and grandpas favorite apps are now “third party only”.
Moot point, Apple's own App Store currently impliments ad tracking pretty much exactly how the Play Store does it: https://searchads.apple.com/
If Netflix decides to open their own store, and Spotify will open another, etc. it will be much more inconvenient for me.
That is not going to happen. It will create too much friction for users.