What Mumble did that was great was getting people to use certificates. I never actually checked the thumbprint of the certificate, but it was a big step forward. I think it could have succeeded with "minimal" changes:

* Have you ever tried to self-host a server, especially on your own Windows machine? Setting it up is a confusing mess, especially the documentation. I couldn't get DBus working the first time I attempted it, so I gave up and paid for TS. Would it have been so hard to include a Murmur configuration dialog in Mumble?

* Ice? Why couldn't a simple HTTP interface work? Just look at all this troubleshooting overhead: https://wiki.mumble.info/wiki/Ice

* UX/UI matters.