@staidwinnow Scale. Capability. Demonstrated malevolence. Demonstrated violation of past pledges and agreements. A history of scorched-earth tactics against previous rivals.
Contact with Facebook by the Fediverse would all but certainly prove violently disruptive if not fatal. Hell, it's very nearly impossible to even discuss the matter without bifurcating the network.
Or if that's too long/didn't for you: the risks are too great, and there's absolutely no basis for trust.
I really don't care to discuss this further with you as this is all painfully obvious and your failure to grasp these points is beyond my own ken.