While politics discussion certainly were a factor in the demise of REHP, the thing that kills online community just like real communities was lack of new blood.
In the early 2000s, TMF seeing their business model of pushing DIY investing was not generating revenue, struggled to find a way to make some $. Noticing that their boards were among the most popular feature on the site and at the time the most popular for investing decided to start charging for access to the forums. The rolled out their charge scheme over a couple years and granted lots of folks a free year. That essentially stopped new people from coming.
The problem with retirement financially planning discussion isn't like cars, or stocks, or football, or American Idol there isn't a lot of new stuff to discuss. (The board does a very good job of scouring the net for new news). So without new topics or new members people end up telling and listening to the same stories. I know I repeat the same stories I try to and do it no more than once a year per story and only in a thread which has some new members. Can't do that without same old crew.
Politics helped convince old members to leave, but the lack of new members was the real of cause of death. I don't spend much time on the Boglehead because they limit the discuss so dramatically. I wish there was more latitude here, but the moderation isn't excessive.
I still follow the TMF Berkshire board, there hasn't been more than a couple of new posters a year for a decade, and very little political discussion. The Intel board which use to get 100 posts a week, I just checked had 89 since I last visited it back in March 2013.