The point of ActivityPub should be to become as widespread as possible so as to proliferate the standard and eliminate silos and walled gardens--including Facebook and its ilk. It would be an unambiguously good thing if Meta's (and Tumblr's) move towards interoperability cascades to the other big platforms like Instagram, YouTube, Twitter, and Reddit.
You could follow (or choose not to follow) users on any platform you want, from any platform you want.
What kinds of extensions should the typical activitypub user be worried about? I don't care if Meta adds payments or virtual avatars or whatever--if the core functionality of the Threads app is simple microblogging, it should be perfectly interoperable with that side of the fediverse.
The more likely effect IMO (if Meta holds to their word on enabling federation on their side) is that other large social media companies (e.g. reddit, twitter) will feel pressured to federate and that will make the fediverse better, not worse.
My account is on kbin.social but I'm working on getting kbin self hosted. When I do, I'll absolutely be federating with Threads whether or not kbin.social does.
Everyone will be over here eventually, including Reddit. Meta/Tumblr joining the fediverse will have a domino effect and will make it very hard for island networks to continue to exist.