[tmarsden]

I'm not an expert here so please take what I say with a grain of salt.

It's my understanding that what's included in open source Android (AOSP) is FAR from a complete product and there is quite a bit of Google closed source/proprietary software that goes into the mix before it's shipped as Android (think Google Services.)

So, while you could fork AOSP and try to use that as a basis for and alternative mobile OS, it would require quite a bit of work on top of the AOSP code. This is what's done by custom ROMs like GrapheneOS (ironically Pixel devices only) or LineageOS for example.

[jhasse]

So it wouldn't require a lot of work because LineageOS has already done the work and is open-source.

[pinusc]

Although it wouldn't require a lot of work to allow side-loading apps on LineageOS and similar, LOS users would still be profoundly impacted by the death of the FOSS ecosystem.

LOS/AOSP/whatever users are a VANISHINGLY small minority of users, so "an app that only works on them" is an app that only works for a tiny minority of people. This would disincentivize developing FOSS apps altogether. A lot of projects will likely eventually die, and a lot that could have started will not.