If that community couldn't sustain itself, is there reason to think a subreddit here would prosper any better?
The problem with discussion of AGI, nanotechnology, and all the other "Shock Level N" memes for N ≥ 2 is that there is no real subject matter. For the most part it's just verbal geekery about cool ideas that no-one is actually doing anything about, because they're too far beyond current capabilities. Fine to engage in for a while at an SF con or in a pub with other geeks, but there's only so long you can be at a party before realising you're just seeing the same ideas over and over and it's time to leave.
I never read SL4 -- is that an accurate description of why it died?
I've entertained a similar hypothesis myself.
As for its relation to SL4, I'd say that it sounds roughly right - I wouldn't go as far as to say that there was "no real subject matter", but it's true that the list eventually ran out of worthwhile things to say that hadn't been already discussed.