Free and Open Source Software

My main point is that all your arguments can feel perfectly reasonable, but they don't seem especially virtuous. Yes, change has a cost and is not easy. But the alternative is simply letting these huge corporations in control. We can do better than that. We can simply start out by refusing to join the larger networks. We can be part of the intolerant minority that ends up setting the course.

if its biggest (maybe only) selling point is privacy and security then I really don’t think most users will move.

No, that is not the main selling point. The selling point is control. Signal may be "private" and "secure", but requires you to trust their implementation and keeps them in control of crucial infrastructure. Matrix (or other open protocols like XMPP) give people full freedom to control how their communications works: it can be a professional hosting company, or it can an enthusiast running in their basement, or it could even be a public service offered by a local government, or it can be an university running their own servers for all students and faculty.

I would love something like Matrix to “win” if it is as good as you say it is,

It will win, at least in the same sense as Linux has "won" the operating system wars. Even if we don't get everyone running Element on their phones, we already have a directive in the EU that will force all major messengers to be able to interoperate, which will lead at least to the larger players to create some set of common functionality that will be supported by the basic phones, and there is a good chance that this will end up being powered by Matrix. There is also the fact that large sectors of the German and French Governments are investing and deploying a lot of their communication systems based on Matrix. Lastly, we can not ignore the fact that even if it's not super famous, there are already an estimated number of 60 million active accounts on Matrix.