One can have metadata for sensitive internal corporate channels stay on a network they own in whatever country they want while still being able to chat with outside parties on matrix.org or other servers. Several friends host their own servers and more recently matrix p2p is rapidly maturing to dump the need for servers at all for many use cases.
Federated systems allow people like me to choose to host a server for my own family in my own home closet rack so data shared between my family and I never leave our network.
Still others can host matrix as a Tor hidden service where it will be very expensive to even learn who to target.
If all participants are using Tor and not using identifying information like a phone number, then bulk deanonymization becomes very expensive, particularly if many small highly targeted social circles roll their own.
Signal does not give users a choice but to trust one SPOF setup built under a one size fits all threat model that flows all IP metadata to one place which can leak information regardless of any encryption.
I don't want a world where one central party holds all communications metadata in one place under one legal jurisdiction with one proprietary memory isolation technology under one threat model.
Moxie here likes to point out that email demonstrates all the things that can go wrong with federated systems, but if had chosen the popular alternative of letting a single party take over this whole class of communication, we could all still be using AOL.
Internet messaging will outlive us all, and if we advocate everyone lock up their communications with one (even benevolent) dictator, it won't end well when the next dictator is not so benevolent.
See: pretty much every social network in China and Russia that is now under state control in spite of early promises of privacy by founders.