Signal is a centralized app, run by a company. If they are offered enough money or legal threat they will sell out or close.
I am sure people will make an argument that its FOSS and people will just fork it if it goes bad, but a new fork will have 0 users and Signal will still have all of your old contacts. Why not make a switch now? Before it is even more popular and you have more reasons to stay? Why fork it if there are already decentralized apps that use same encryption, like XMPP apps?
Sure you can find flaws in every app, including XMPP implementations, but if we will have to write code for a new Signal fork, why not just fix whatever is that bugs you in XMPP clients?
If you want to use Matrix, that is fine as well, we can always bridge the two open protocols. But you cant bridge Signal if their company doesn’t allow it.
See the difference is, you need to convince non-technical people to use xmpp. Most of my non-technical friends already have Signal, no convincing required.
Combined with the fact that Signal has an extremely reduced risk profile in terms of data stored by Signal and the hassle of either setting up my own xmpp server or trying to vet one that is trustworthy for the increased amount of data that is entrusted to server operators with xmpp compared to signal just makes it a non-starter unless I want to use it with other techies who are already game.
Sure I could also convince all my non-techie friends, but that’s a lot of work for practically very little privacy benefit.