As long as the iOS model exists, the sideload store model is only viable on Android, and vendors are forced to support the first-party store model anyway. If both ecosystems allow sideloading, you could easily imagine Microsoft or Epic switching to sideload-only and branding their own stores across Android and iOS. As it is now, if you can get something first-party on iOS but are forced to sideload on Android, it just makes the Android experience for Fortnite (or whatever) seem janky.
This is a good point, though the lack of auto-updates from non-Play stores on non-rooted phones does add enough friction to updating that e.g. Signal won't even distribute via f-droid because of update latency. At least that's my reading of Moxie's reasoning. It seems likely this would dissuade some companies from making their own app stores, though obviously not all.
