That's a very charitable assessment. The problem is the oversaturation of everything with politics (politics is the lowest-common-denominator subject that everyone can argue about). The loud politics minorities are so numerous now that they can easily sway the attention of even niche communities. You can see that most glaringly on reddit where pretty much all important conversations are destroyed by becoming the same old politics debates, like how /r/technology is politics or r/coronavirus is politics . Lots of people stay in HN exactly because it moderates against politics but HN exists because YC doesn't mind running an unprofitable site.
Most forums shut down because they are unsustainable, and the reason they are unsustainable is that the current mobile/social media vortex is hoovering all the attention. This leads to a negative spiral where people don't make good self-hosted forum software / community software anymore and so on.