This comes up every time, I just don't understand the argument. You can fork AOSP when that happens. It will still be far less work than bootstrapping a viable open source alternative from scratch. Also maintaining a forked AOSP would be less work than getting the Linux desktop stack on the same level as AOSP.

Given that Android apps target old API levels, it would also stay compatible for years and nothing would hold you from implementing APIs from the docs.

To be honest, I don't think AOSP will fully close up, since Google probably fears the regulatory pressure and the fallout from vendors. The larger risk is Play Integrity, but that applies equally to other Linux systems if you want to tap into the large Android app ecosystem. But the solution for that is not technical, the EU and other regulatory bodies should make forbid it in its current form due to anti-competitiveness.