I want to add "federated" to your list, as the only thing that actually matters long term. Signal checks your requirements but has already started to turn user-hostile (e.g. it mandates its own client so you get to have crypto payments whether you like it or not), and, as the single point of control, is an easy target and a single major liability.

Remember the days when WhatsApp was nice to its users? There is no technical guarantee that other centralized systems won't go the same path, which is largely mitigated when the network is made of smaller interoperable actors (i.e. a federation).

I would love to see XMPP be rediscovered and massively adopted as that next gen messenger. I don't trust Matrix to ever be reliable or get past their neverending funding troubles.