> then try to find your contacts again (which may be cached by your client or not)

You can save XMPP account IDs in any mobile address book.

> Finally, you have to convince "someone" that this is just you with another account on another server. There is also no verified E2EE anymore.

I'd pretty much just do a video call at that point.

> How is this different from "when Signal goes down one uses a completely different instant messaging system", apart from using another client?

You don't have to use another client software.

> Which means: If one of these hosting companies blocks XMPP traffic (e.g., if a rogue state starts censoring) or one of these XMPP servers goes down, a huge part of XMPP users is affected.

Equally applies to most messaging apps out there. Some XMPP apps have the benefit of also supporting using Tor and hidden services - Signal/WhatsApp/etc, don't.

Don't want to use the Internet at all and your friends live close by? You can even go wild and do something like:

- https://github.com/ddamianus/Lora-Chat-Device

- https://github.com/jgoerzen/ax25xmpp

I realize this is something most people would not do, but XMPP's _flexibility_ allows this to be an option if it was something people wanted. Can't do Signal over LoRa.