I think many people here are immensely overestimating the value of the Fediverse user base. The entire active Fediverse, let alone individual instances, is barely a rounding error for Meta.
There is no if or when Threads become the biggest instance, Threads apparently got 10 million users in 7 hours. The whole of Mastodon has ~9 million users in total. By now, Threads alone is likely bigger than the entire Fediverse combined, which mind you is something like >99% bots and inactive users.
Even if every single instance defederates from Meta, their fork of ActivityPub would by far be the most significant one by not a small margin.