Hello, Peter
you bring good points into discussion.
I also think that people has the rights to complain, but is always better if you can think a little more about things and propose something useful to do about =)
About this forum being sustainable over time, well, time will tell us =)
I do not work for Moodle HQ, but have fixed some little pieces of moodle over time and having a place to discuss with other people before suggest anything on tracker is good =)
In Moodle Development Process (https://docs.moodle.org/dev/Process):
"All issues on the FRONTEND and BACKEND backlogs are given a relative rank that is based on issue features including priority, security, Partner interest, patches and votes."
The bold was my addition as i think this is the ones with more influence in deciding what will be worked on by Moodle HQ. Of course everyone else (outside) can contribute along the process and get unrelated-new-features added too.
With that in mind, i think this forum is a place to say that we want to get involved as community =)
Kind regards,
Daniel