I haven't thought much about this, but given the historic failure of alternative app stores to gain traction in Android, what makes you think this will change? Hardly anyone I know outside of tech circles (and some within them) are even aware that alternative web browsers are an option for their phone, let alone alternative app stores.

I think best case is sites like itch.io and Patreon have dedicated app stores that funnel more money to creators and it's easy enough to manage having multiple active app sources on your phone, but they will always have a very small niche market compared to Apple or Google. Even Amazon hasn't been able to eat away at the defaults' market share.

The most viable path I can see to getting alternative app stores as a common scenario and user expectation is basically just... Steam. But mobile gaming is such a flaming shit show, I don't know if that will ever happen. Something like Steam getting traction on smartphones would absolutely help change that problem though.