I get the impression that some people basically do want a copy of reddit, just one that hasn't yet reached the critical mass where it's too hard to see through the shitposting to the substantial discussion. I don't know if they realize this is a temporary state, or maybe they think they can keep it from happening if they get in as one of the vanguard.
For the most part I only see two ways to avoid Eternal September: 1) Stay small and/or incredibly focused. This is effectively security through anonymity, but having a strong central theme with good moderation can stave off the seemingly endless shitposting. 2) Flee the sinking ship. Once a site starts to go downhill, there's really very little chance of it truly recovering. The people running the show either think it's a good thing (Yay, more traffic!) or they're asleep at the wheel, which should shake your confidence.
As long as a site stays pretty strong when it comes to shitposting, it should be fine. When looking at the defaults, there's a lot of shitposting in subs like /r/funny and /r/askreddit, but if you go somewhere heavily moderated like /r/science or /r/askhistorians, the discussion is pretty stellar. The key to keeping good discussion flowing is heavy moderation. If a site stays too loose, then the whole thing keeps getting filled with people posting the same things over and over again. However, with the right amount, it's possible to create a site that retains good discussion without getting too overzealous with moderation. If it lets people moderate themselves, it works pretty well, until the shitposting gets too much to filter out on a user to user basis. At the same time, if a site doesn't reward people for regurgitating popular talking points and punish people for giving dissenting opinions, then it fares a lot better. 4chan is full of shitposts, but simultaneously has good points being brought up, regardless of the consensus (exceptions: /b/ and /v/). There is no upvote to push things to the top, nor is there a downvote that hides things to most users - it's all equal through the eyes of the ranking algorithm. Also, there's a lack of point tracking system that pats people on the back for contributing towards the hivemind.
I'll come back later to properly reply to these posts, if I think of a good point, but for now I wanted to point this out. On the chans, there is a vague analogue to voting in that, if you reply to a thread, it bumps it to the top as usual, but you can sage to reply without bumping. In this way, channers can flame a shit thread without giving it the bump it doesn't deserve. It's not really the same, though, not the way it is on reddit, where you can actively suppress content with voting.