I suppose the part I seem to be missing: what's to stop Facebook from setting up a "Signal Server" that then hosts those users on Whatsapp/Messenger?
From there, what happens if Facebook then attempt to integrate those servers into the existing Signal network?
I'm really not sure how information is shared between servers on Signal and am curious if there's not something at a purely technical level to stop that from happening. I'd imagine there's some keys that need to be passed around for handling en/decryption which I think is what you're alluding to, but I want to be clear that that's what you mean.