I just decided to pop in after a really long time and discovered this thread and thought I would give my 2 cents
The decline of HJU is really sad, but I think there are multiple factors that caused it. The biggest one is that social media has taken over forums, most of us joined in the mid-2000s when stuff like Twitter never existed, and we just wanted to find people who shared our interests and HJU was one of the best ways.
I think most people who watch Toku are just getting older, when you're in your mid-late 30s, work 9-5 and have a family its tough to be active on a forum. The current way Toku is being made doesnt help either, the last full Sentai ive seen was Gokaiger and I have no interest in watching new KR or Sentai because both shows have become 22 minute toy commercials, we will probably never see storytelling in a show like Kamen Rider 555 (for example) again.
I miss the community of HJU and forums in general, but they sadly represent a relic of the early-mid 2000s that have been replaced by something new