I read a lot of comments like this, saying "we should have an alternative". And I agree that it would be really cool, BUT:

AOSP and the Android SDK are open source. GrapheneOS is exactly an alternative to Google Android. Sure, Google could shut down the Play Store (but alternative stores exist), or destroy the Play Services (but even then, there is microG).

I am happy that Google is still contributing to Android because they have a ton of money and many brilliant engineers, even though as a company it seems like it's somehow trying to burn it to the ground. But I am optimistic: it would be possible to build on top of the open source parts of Android if needed.