Earlier quoted context omitted.
That's already happening today. https://grapheneos.org/ But for the reason an antiquated os like postmarketos are suggested is that the project is being opportunistic thinking this is a chance they can be relevant. Additionally, the population of HN has more sentimental view on these legacy operating systems and view it as a chance to go back to the past and use software they are familiar with.
> That's already happening today. That's not a hard fork. They always rebase on top of AOSP when there's a new AOSP source release
Regulators in the US decided that Android did not have to be split from Google, but they could theoretically decide that Google is not allowed to break AOSP to gain a competitive advantage. Not that it would matter: TooBigTech is too powerful to care about regulations anyway.