Mainline kernel tends to have only basic support (if at all) for many SoCs that actually get used in phones, especially full power management support has been lacking.

Right, but it doesn't seem SoC vendors are budging on that - maybe it's slowly time for Linux to figure out a better approach?

Most PC and server hardware has FLOSS drivers (with proprietary firmware), even Qualcomm is upstreaming support for the new Snapdragon Elite (maybe it's made by a different team?).

I think phone SoCs are the odd ones out, which sadly doesn't mean they'll improve any time soon. Supporting an ABI for binary drivers in Linux might help phones, but it would give everyone else a chance to regress in their support, so I understand Linux kernel developers' position.

Stopping planned obsolescence via closed drivers is a start, no?

Linux isn't budging - maybe it's time for SoC vendors to figure out a better approach?

Android can boot from a mainline kernel, but all that gets you is that phones run a kernel forked from mainline, not forked from Google's fork.