there are plenty of "real" open source mobile OSes. the issue is third party apps. if Debian couldn't run NGINX, Apache, Docker, Kubernetes, etc, it wouldn't matter how mature or solid the OS is, because 90% of software people want to run on a server doesn't work.
if FAANG apps and banking apps don't run on a mobile OS it will never be viable. the government, these big companies, and the device manufacturers all have a vested interest in making sure it never happens.
Honestly, at least for me, it doesn't have to run those things. It has to run a private browser and Signal, and it needs to run on easy-to-obtain smartphone hardware with no dependency on anyone's app store. I would hope that is at least somewhat doable.
I have no delusions about there ever being a year of the GNU/Linux smartphone. Google will make damn sure that never happens, like you say.
EDIT:
I should say, I see this being a "second" device. Something to use when you don't want someone generating profit off of your data.
if FAANG apps and banking apps don't run on a mobile OS it will never be viable. the government, these big companies, and the device manufacturers all have a vested interest in making sure it never happens.