If you ignore GitHub, how many of the problems described go away (and of the ones that remain, how potent are they)?
The problems are not intrinsic to "open source". Conflating GitHub culture with open source is like conflating being on Twitter with using the Internet.
* The larger presence of commercial OSS projects setting a tone of "projects here are products to be marketed"
* Social media-like features encouraging popularity contests, curating a personal brand, etc.
* Both of the above leading to more users acting like you owe them something ("you're competing for my attention, right?")
I'm not 100% convinced that those or other GitHub-specific factors are primary causes of maintainer burnout, but I think it's certainly possible.
Neither. Everyone and everything develops a culture and/or a set of norms. This is the one that emerged on GitHub. Looking for the reason inside the machine is misguided. It's the people.
> Anecdotally I see many of the same problems elsewhere (GitLab, and even independent/self-hosted)
Putting your repo and bugtracker somewhere that isn't github.com isn't sufficient to neutralize the GitHub culture. (Do people from country/culture X stop being affected by it when they go off and spend the summer in country Y?)
For many or maybe even most people doing open source on GitHub today, they're not even going to know which direction to drive towards in order to achieve the norms of pre-GitHub culture because they never experienced it.