I don't think that's the problem. AOSP is free-software but Google tries to enforce strict guidelines for "Android" labeling. It has some good sides (Treble architecture for maintainable devices) but mostly bad sides (forcing to package the Google Play Services universal spyware). Still, whether approved by Google or not, you find plenty of both high-quality and low-quality Android distros (with and without source code), but Google's approved Androids certainly have much less fragmentation in terms of features/settings than competing (or homebrew) Android.

> it would serve System76's brand to have a tight Ubuntu partnership/license

How would that benefit users? Canonical (Ubuntu's editor) is basically the Windows of GNU/Linux world, forcing telemetry and enabling privacy-invasive ads into your desktop. Actually-benevolent entities like System76 could only benefit from dropping Ubuntu entirely and contributing directly to upstream Debian (the basis of Ubuntu) which is maintained by a non-profit foundation to which many different entities (including other big companies) contribute.