This sort of thing would be a lot more convincing if, you know, it wasn’t just Discourse. Best practices for handling obsolescence are already in practice all over the web. Discourse’s attitude of either “full support or HTML only uselessness” has NEVER been the standard. HTML and JavaScript were designed around graceful fallback.
It’s not some inevitability that they have to deal with. If it were, you’d constantly be running into websites saying “sorry, we can’t work on your browser.” It’s a choice. They don’t have to constantly rewrite everything to use the latest and greatest.
It seems unlikely every other website is weighing the technical debt wrong. Ripping off the bandaid seems a horrible strategy in general (as users resist forced obsolescence), and especially so in this case, as message board users skew older and thus are more likely to use older hardware.
I get some hints of choice supportive bias when people defend Discourse for things that other websites and webapps don’t do.