No, there is also a question "how long can this messenger stay secure?" In case of Signal, IMHO, the answer is "not very long".
Would you like an example that's especially relevant to this thread? Because I can provide you with one.
I never mentioned any particular kind of federation that I prefer. I also never mentioned word "private", which is mostly tangential to federation.
My point is that, in the long run, only federated systems are sustainable, because nobody is able to fund huge servers with millions of users without infinite money. For example, Signal and Telegram are both struggling with that currently; the latter introduced advertisement recently. On the other hand, Mastodon does not seem to have such problem (you could argue that it's too early though).
I do not want to build my own clients, I'm not even a programmer. I want a sustainable network, which could be achieved by a large number of self-hosted instances federated with (possibly) large servers. It has been working fine with email for decades. Also, the competition of various independent clients improves the user experience (like any other healthy competition elsewhere).
"Sustainability" is arbitrary. Everything burns eventually.
I am mostly talking about a use case, where you try to switch all your friends and relatives to a new IM system. If you have to ask them to switch again in a couple of years, you will loose their trust quickly.
That doesn't make Matrix bad. It makes Matrix different. Matrix has different goals than Signal, and those goals simply prioritize security and privacy differently than Signal does.
You're completely right that we have different goals to Signal (openness + freedom rather than privacy-at-all-costs), but I'm not sure that blaming federation is the right answer here. We just prioritised building an open standard over having E2EE from day 1, instead choosing to design things so we could add it later.
Again: I'm not dunking on Matrix. There are things the Matrix approach will do better than Signal can. But protecting individuals is probably never going to be one of them; you're competing with a project that has security and privacy as its overriding goal, and nobody at Signal ever has to ask whether federation --- a protocol complexifier if ever there was one --- merits a security hit.
I don't use Signal for everything (or even most things). I'm completely open to the argument that other messengers are better "overall" than Signal. But on a thread that asks the question of whether Matrix is as secure as Signal (not "secure enough", but rather "at parity with"), there is simply a clear answer.
I'm willing to believe that this is true for Telegram, since they do pretty much everything on the server. With Signal, though, message databases and most business logic are client-side, so the servers are surprisingly dumb and lean. Signal spends way more on salaries and wages than on its servers. In 2019, Signal paid more to Twilio for SMS verification than it did to AWS for hosting: https://projects.propublica.org/nonprofits/organizations/824...