These OSes exist today, and I'd like to use them, but they're typically limited to a very small handful of (typically obsolete) devices. As long as this is the case, they'll have a lot of trouble finding enough developers and users.
If we could simply link our own kernels against these drivers the way we do with the nvidia driver and DKMS on desktops, it would be a revolutionary step forward for alt smartphone OSes. We could start working on userland instead of spending so much time devising hacks to simply get graphics and radio working one handset at a time. Unfortunately, the OEMs couldn't care less and would much rather our phones become e-waste than eat into next year's sales figures. Until then, we're pretty much at the mercy of Apple and Google :(
There's Ubuntu Phone, KDE Plasma, Copperhead and others but they don't feel like a good solution. The problem is always the same: no apps. It doesn't matter if I can install 99/100 apps I need but the one I can't is a messenger one of my clients requires me to use. Or if I can't really rely on a compatibility layer like MicroG, since Android is not that open anymore.