That isn't a reason to push against Signal, it is a reason to push for a federated solution.

Not your point, but federation is often heralded as some must-have feature for communication around here, but we already have that: XMPP does that, even with encryption. Or Email with PGP. Or even the old Textsecure code, as Signal/Textsecure started out federated, but, surprise, that didn't help adoption.