If anything, LW is far more at risk of becoming an echo chamber than of an eternal September.

Do you have any particular reason to believe that? (many people show opposite concerns, and more specifically the stats seem to show increasing numbers of new users)

there's a fairly steep learning curve before someone is accepted here ("read the Sequences!")

That doesn't happen that often, and when it is it's usually recognized that it's a bit of a cliché. The case where it seems most likely to happen is if a new member starts proposing some great "new" ideas on AI or friendliness or morality, such as "it's obvious, for Friendly AI all you have to do is tell the AI to be nice!".