Wouldn't the answer for most people to "What happens when Twitter shuts down?" be, "Well I'll just move to the next social media site".

I don't necessarily hold that opinion, but I get the impression most folks I know do.

people will move to the next social media site (or jump to another medium if it's good enough) even before Twitter shuts down.

This happens all the time and there are parallels with other format / medium shifts (Gutenberg invents movable type, newspapers supplanted by news reels at the cinema -> people buy radio receivers -> broadcast television -> cable news -> whatever we have today with our always-on internet connections and services.

My main point here is that audiences are fluid, we can respect their intelligence, and they go where the content is.

>Bingo. I say this all the time, Twitter is not immune from being a member of this list (Defunct social networking sites, wikipedia):

It is if people who've decided it's "de facto infrastructure" get their way and the government nationalizes and regulates it. Then we're all stuck with it forever.

nothing's stopping your local city council, library, or fire department from shoe-horning the ActivityPub protocol into their existing website (like WordPress or Drupal or whatever CMS they use) and immediately turning their site into their own Twitter service.

I'd like to see RSS come back in a big way, but replace RSS with ActivityPub and instead of nationalizing shitty centralized commercial services, adopt the protocols that allow for distributed and federated social activities.

Sure, but commercial entities generally shut down when there's too few users to justify the costs. But then you're not comparing the value of current Twitter shutting down but an empty wasteland Twitter shutting down. An empty wasteland Fediverse also wouldn't be of much use to the vast majority of people.

By same logic, UUCP never died.