We're starting with the ability to follow threads users from activitypub clients, but we will get to the ability to follow accounts from activitypub servers on threads as well

If 2. will actually be a problem some instances will defederate, while many users will choose an instance which allows them to follow who they want. I'm all for interoperable social media/messaging, because it gives users the choice.

I'm curious when they'll add inbound federation. It could lead to massive amounts of spam, so they'll probably block instances or inbound traffic quite quickly.

Hopefully it won't end like email, where it's really difficult to start federating to the big providers (Threads). But even then, we'll still be able to choose any of the current instances and continue without them. Edit: It's not a big problem if Threads doesn't show all posts, since other instances will still show them to users who care. Compared to email where a 100% delivery rate is critical (at least for important stuff).