It's pretty much the ending that kills Decade for a lot of people. I started getting into Rider just as Decade was winding down and I remember it being really, really popular at the time. It's just when the movies came out and answered absolutely nothing about the show's characters or plot that most people turned on it.

It's probably a case of the show switching production staff partway through. The original writer set up a lot of mysteries, but the new writer cared more about cramming in as much fanwank as possible instead of developing the actual original characters and story. That's why we started getting stuff like Dai Shocker and the Showa Riders instead of addressing Tsukasa's original world or what Narutaki's deal is. Sadly, that writer is still doing this shit with the Super Hero Taisen movies.

Matrix has nice things to say about Decade? *looks out window* well...the world seems normal...

The impossible happens sometimes...