I'll bet upstream is more-or-less welcoming of an additional (well funded?) stakeholder. The bug reports alone might be worth the effort, especially if System76 is triaging some.
The number of FOSS contributers is not infitine. At some point we need to start working together instead of forking yet another marginally different copy.
I understand some diversity is good, but what popos adds could have been added to vanilla Ubuntu (or Debian, if you feel that way) and benefitted many more.
To address your original point, I suspect it's more expensive and slower (at least, in the short term) to add features to someone else's distro than to fork it and keep building it out yourself.
Nothing. Everything we create here at System76 is open source, either GPL or some form of permissive license.
(Edit: To be clear, I'm just answering your rhetorical question, not insinuating anything about the tone of your post for sure.)
I just figure Canonical probably has a specific vision for Ubuntu (just like System76 surely has for Pop!_OS), so they don't necessarily want to pull in every feature from every downstream distro. The prevalence of purposed-focused distros is one of my favorite things about the Linux ecosystem.
I actually haven't used Pop!_OS yet, but it looks good and it's on my short list of distros to try. I didn't mean to imply anything negative about it.
(/s)