Everything is centralized because of one simple reason: you can't monetize distributed.
So, as a developer, if I want paid, I'm going to make a centralized system. The fact that the centralized system is way easier to develop/debug is just a bonus.
As a user, I'm going to use the system that has the easiest installability/usability cross section. That app is likely to be the one that gets the most developer time. The one that gets the most developer time is likely to be the one that lets the developers get paid.
And thus, the circle is closed, and the feedback loop begins.
You can monetize a fancy client that's commercial.
You can monetize hosting Mumble servers.
Decentralized services are much harder, but at some point we have to prefer them and use federation as a means to have something like a global network of continents of servers (or realms in MMO lingo).
My point is that as long as we don't pursue decentralized services, we won't get them. There are efforts for people to host everything privately, and with ipv6 I like to think it's only a matter of time until all IoT devices can be repurposed as servers.
Distributed means spam. That's not a solved issue. Email sorta gets around it with tons of filtering. On IM, spam is much more annoying (it's assumed you'll get lots of junk email even if not spam).
In what way does a centralized server prevent spam? You'll only receive messages after you've accepted the contact request, so the worst that can happen is contact request spam.
Everything is centralized because of one simple reason: you can't monetize distributed.
So, as a developer, if I want paid, I'm going to make a centralized system. The fact that the centralized system is way easier to develop/debug is just a bonus.
As a user, I'm going to use the system that has the easiest installability/usability cross section. That app is likely to be the one that gets the most developer time. The one that gets the most developer time is likely to be the one that lets the developers get paid.
And thus, the circle is closed, and the feedback loop begins.