It's not surprising really, is being a centralized message bus really attractive when it just means that you'll be responsible for policing it?

If you run a central message bus off which people can produce their own applications, you'll find yourself having to deal with everything that ISPs and mail providers have to deal with. That means being asked to keep logs for years, dealing with DMCA takedowns, filtering the worst of the internet, etc.

And for what? As soon as anyone relying on it achieves scale they'll re-implement it themselves.

Besides, what you've described is the opposite of a protocol: an inner platform.