So AOSP would survive fine after stopping taking in new code from Google?

What scenario is this?

If AOSP is suddenly the only acceptable smart-os on phones for 600 million people, I think it would work out yes.

Isn’t that the scenario in this post? Cutting all ties with US companies. So they would stop using code written by Google. Isn’t most of the AOSP code written upstream by Google?

That might be, but with hundreds of millions of paying customers there is a market, and it will be filled. Maybe by some of the tens of thousands of European developers currently working for Google in Europe, or the other American companies.

Continuing a already existing open source OS is far down on the list of challenges.

AOSP is mostly licensed Apache and LGPL. The repos have probably been cloned a million times. Although Google has taken large swathes of the project (like the launcher or Settings app) behind closed proprietary doors. But that is not a huge obstacle, there are already plenty of decent open source launchers.

The most important thing would be a (drop-in) alternative for Firebase Cloud Messaging. Without it, you can say bye bye to any decent battery life.

They "just" have to make a phone that can be supported by GrapheneOS.