> I recently set up a Mumble server on my home server

That makes you an even smaller minority unfortunately. Most people are not going to set up a home server.

It's semantics, really. Run the server executable on any computer in your house. Done. If you don't leave your computer on 24/7, the Mumble server isn't up 24/7. Oh well. If you use it to talk to friends, you'll presumably have your computer on when you want to use it yourself anyway.

Mumble servers used to be just a few dollars a year (haven't looked in a long time). But even that is enough friction to prevent adoption. Plus it doesn't support any of the non-voice features.