You’d also break compatibility with the previous version of your software, so your users wouldn’t be able to see any threads posted on any server that wasn’t updated.

If you had a really good reason to make this change, chances are this reason would apply to both services and they would move together. If either developer does it out of spite, nobody is going to upgrade their instance and the project would be forked.

And if they were the type to break ActivityPub integration of their service out of spite, why would they use it in the first place? It just makes no sense. I’m not particularly worried.