I don't think it's just an advertising/cancel culture problem. It's partially a problem with the internet being so decentralized that spammers and scammers can operate with impunity, pushing a ton of work onto people running online communities because countries like India and Russia don't properly go after cybercriminals - even if someone SaaSified forums, this is still a significant burden.
It's also kind of a UI problem: if Google search starts to suck and gets filled with trash, as has happened, it breaks the core pull-model of the web. If Reddit starts to suck and get filled with trash, as has happened, it breaks the core push-model of the web. Together the two kind of Embraced-Extended-Extinguished the web, but maybe something like an RSS reader, different web client (eg a browser that didn't nudge you into making google queries as the main way to operate it), and more capable search engine would be able to fix these.