The thing is: None can compete with the Discord in its core features. They either have good chat or good voice, but not a combination of both.

Discord replaced Ventrilo, TeampSpeak and Mumble simply because it merged Slack/IRC-style chatting with an easy to use voice chat that took less resources than skype and less setup than any of the self-hosted version or cheaper than a hosted version.

Low-latency voice chats combined with Slack, I don't know why no one has an alternative for that.