The argument for defederating is that Meta has an enormous technological and userbase advantage for capturing up all the activity in the fediverse. It's not out of the realm of possibility that the overwhelming majority of future activity on the fediverse happens on Meta controlled instances, if we let them have free reign capturing as much of the fediverse as they can. In that case, with Meta effectively controlling the fediverse, then they don't really need to play nice anymore. They can introduce a breaking API change and hold all of the non-Meta instances ransom saying to upgrade to their new API, or you won't be able to participate with their fediverse communities anymore.
So it's basically a question of do we nip the Meta issue in the bud and preemptively defederate from them, or do we wait until they take over and force us to restart from scratch two years from now.