With google following apple’s walled garden, and limiting third party app installations, can someone else big (nonprofit like GNU or Linux foundation) fork and maintain android? Reason for choosing someone big is for mass adoption

Could that happen? No. A massive amount of android development comes from employees paid by google to do it. What amount of resources should be siphoned away from linux/gnu stuff to support android developers? None. Is it possible though? No. Android is a proprietary binary blob core (idr if kernel is the right term) with a bunch of open source stuff wrapped around it. For gnu, that part would have to be rewritten and that’s too big a job to take on. Should it happen? Again, no. There are already plenty of alternatives to google branded android. Just use those. Even if you were to wave a wand and make the android custodians according to your will, play services, the thing google is restricting, is still googles thing.

The second paragraph isn’t accurate - Android uses the Linux kernel. Hardware manufacturers typically ship their own proprietary modules, but in principle there’s nothing stopping you from running a mainline kernel image as long as support for the specific device or SoC is present. Granted, this isn’t the case for the majority of phones, but this isn’t a limitation of Android itself and you’d run into the same issue trying to run postmarketOS or really anything else on an unsupported device. I also want to point out that there really aren’t any good alternatives to Android (and its derivatives) at the moment. Mobile Linux distributions are getting better, but at least in my personal experience and from what I’ve heard from others it’s still a pretty janky experience in comparison and arguably is only suitable for enthusiasts.