It's not that seo has taken a nosedive ( I guess, it does depend on whose perspective ) More like, it's become such an hyper-focus on it that we have a bunch of seo-driven drivel destroying the quality of the results.
People mostly figured out how google wants things to rank them high. So they do only that. And a lot of it. Anything that doesn't gets relegated to further down results, or, gasp, the second page. Which means a loss in revenu, so you kinda --have-- to do it, business wise.
I think you have a good point here. The type of user that migrates. Plus, since the api changes makes moderation harder, there might be a bunch of mods moving over, whom would tend to have a greater impact on their communities ( may it be more activity, or modding, etc )