They give pretty good reasons for what they're doing on their website: https://postmarketos.org/blog/2017/05/26/intro/
Which has finally driven Google to change their Play Services access contract.
https://postmarketos.org/blog/2018/06/09/one-year/#mainline-...
Android can’t be trusted anymore unless major change of policy. By extension AOSP are better but still dubious since they can’t totally cut the cord from Google if needed (as far ad I understood).
Without taking away anything from your argument, I'd like to point out that by most objective measures, Android is the mainstream Linux stack.