> An alternative to the Android-iOS duopoly that involves futzing with cables and command lines is never going to go mainstream

All you need to install graphene is a web browser and USB cable.

> By contrast, a vertically integrated FOSS(ish) hardware-software stack might eventually get to 5% market share, like desktop Linux

Those exists. They've never gained traction. If you think graphene is too much tinkering, you'll be traumatised by that landscape.