thingerr
Secondly, AOSP isn't forever. After your carrier/device-manufacturer drops support, Google isn't that far behind. If you're not getting AOSP drops for your device that work with device drivers then it's probably dead. There are a few brave souls willing to port modern Android to no-longer-google supported devices, but I wouldn't call that sure-fire security.
Thirdly, many AOSP derivatives and communities have niche motives that don't really align with the average user. New OS features, experimental "battery saving" kernel hacks and user-space root are commonplace where they really shouldn't be for the average, or arguably any, mobile user. Often devs get a new device and the community quietly moves on, dies, stops providing.
Mobile software is really in a sore place right now overall and neither duopoly is 100%. The incentives aren't aligned to the consumer.
It's true that, practically, most companies won't ever have to worry about Facebook infringing their patents, but I think it's garbage on principle to reserve first-strike litigation while making it impossible for someone else to do the same and use software you've made available under a supposedly permissive license.
[0] https://github.com/facebook/react/issues/10191#
[1] https://code.facebook.com/posts/112130496157735/explaining-r...