I don’t think it matters that much. The users are very different. Most Threads users will be Threads users, regardless of what the fediverse does.
In the contrary, if a Threads user can interact with the fediverse, he/she could change his/her mind at some point because now he/she will be aware of the fediverse (eg Meta starts to put a lot of ads, many users will migrate towards the fediverse instead of paying Meta a fee).
As a consequence, I think Threads will stay « friendly » with its users in fear everyone migrates somewhere else. Otherwise, no one will leave Threads unless there are major issues (like Twitter or Reddit). Not federating because we fear the fediverse will not grow as much as a consequence is, in my opinion, the exact opposite of what will happen.
Just look at YouTube alternatives. People won’t leave the platform for peertube because they would lost all the content. Now, imagine that YouTube was part of it, do you really believe people would stay and endure as much ads ? I believe they would leave and YouTube would be forced to refrain itself in order to keep its users, and peertube would become much more popular.
TL;DR I don’t say we should federate at any cost tough. But I don’t believe the fediverse will grow because it rejects Threads (in fact, I think it will be the opposite). The question is more something like « Do we want the mass to be part of the fediverse ? » (with all the consequences like brands starting to put ads / communicate here, and a bunch of racists & cie that could possibly be impossible to moderate).