you’re supposed to write a risk assessment

Risk assessments are trivially banal.

if I were running a site as a hobby I wouldn’t trust my own assessment

I’d trust mine.

if it turns out you’re multi-risk then it looks like you need to be sure all sorts of measures are in place, some of which may require building capabilities that you don’t have yet

The measures that are required are the kind of things which now seem basic. Like having a means to flag posts as problematic. If I ran a forum that didn’t have such functionality, I’d be concerned regardless and probably take advantage of the kick up the arse.

I don’t blame anyone for deciding it’s not worth it.

Well that’s the question isn’t it. Is any particular forum worth bothering with. Clearly the cycling forum that’s shutting down isn’t. But I question whether that’s due to the new legislation.