This is a great point made later on the thread.
And already happened in some countries, for example, with whatsapp.
Social pressure was big enough to revert it in a reasonable time, but time is very relative on how much you or your business relies on it.
While when your XMPP server admin does something weird or a country breaks it you are locked in.
For Signal you use an ID which is not owned by Signal. So for Signal and alike the decentralisation doesn't come from the service but from your own social graph which is your phone book on your smartphone.
Moxie explained it much better I think. https://whispersystems.org/blog/contact-discovery/
I suspect that relying on IDs owned by another system can have its drawbacks. Maybe it's not likely that an entire country is forced to change phone numbers. It's still possible to lose a number for various reasons (most entirely benign). An oppressive government could definitely disrupt communication of suspects by forcing the phone company(-ies) to terminate their contracts and unlist their numbers.
(But IDs being piggy-backed to a different system which is not easily disrupted on a mass scale does have some advantages, too.)
I got my mobile number in 1999 and already switched twice with the number. Sadly it is kinda expensive to take the number with you, 25 to 30 euros.
I'm sure that if Moxie turned evil it would be difficult for the project to survive, but that's also one of the reasons they've been successful. They have control over what they're building, and they can move fast and make breaking changes. It's very difficult to build something beautiful when you can't make breaking changes.