I think that it is a bit premature to speak of an 'always' when we haven't had more than a few years of experience with any of this. It may take some time to get it right, and as long as the lessons from the failed communities stay learned there should be some visible progress.
HN is an excellent example of progress compared to those that came before. It almost certainly won't last forever but whether it succeeds or fails it has already raised the bar for what is possible considerably.
"Eternal september" isn't really new. We've had nearly 20 years experimenting this, we've seen usenet declining, slashdot, digg, reddit and whatever other net community you want to. Even HN is on the downward slope nowadays, I'd say.
What's interesting in this analysis is the second drawing. It looks like one of the best "well kept garden" out there, Perlmonks.org ; it never devolved thanks to its users and moderators dedication, but it's probably losing steam and users to stackoverflow and al., I think because of its 1999 look and feel.
HN is an excellent example of progress compared to those that came before. It almost certainly won't last forever but whether it succeeds or fails it has already raised the bar for what is possible considerably.