You bring up a number of important points. Perhaps I missed this when reading, but one role LessWrong plays and continues to play is a good source of discussion. Often I'll find the discussion to be more interesting than a particular article. It's not uncommon for me to be linked to a particular comment divorced from its larger context and not be interested in the larger context. I don't know how common this behavior is, but this is not uncommon for me, and I don't think replacing the rationalist materials with a wiki or Q&A site would suit this well at all. This is one reason to favor something like Reddit.

I'm also generally not a fan of shutting down even semi-active forums. In one online community I've participated in, there were several major forum closures, and each time there was a period of confusion about what to do if you're interested in discussion, along with basically sectarian posturing to get active posters. The sectarian stuff caused major problems down the line, and the current discussion forum for this community more or less voluntarily avoids those conflicts now. There also are a number of roles LessWrong plays that I'm not sure would survive a transition to the diaspora, like the page about sharing academic papers. I also often enjoy reading the open threads. Perhaps transitioning LessWrong more towards discussion would be a good middle ground.

Edit: On a related note, I find following discussions on Tumblr to be a huge pain, and hope either this improves in the future or that more discussions happen elsewhere.