Title. There really isn't a good reason not to release it to team wide. DoTA 2 has had this for years now, and 9 times out of 10 there isn't a problem, and when there is you just mute the person. Much like regular text chat, if people are being toxic you can just mute them. It was mentioned by Soundwave that there isn't a good option to hold back toxicity, but how much more is needed than a mute button? DoTA tracks reports across games, and they're pretty quick to mute players that are abusing people in voice comms.
It's really silly to see "We'll be working on voice comms" and then 2 sentences later you say "It'll only be available to pre made parties". Whats the point? I've got a discord server with everyone I play with, I can put spotify on for us to listen to, there's great video call options, and I'm going to ditch this for plain old voice chat? It seems like a waste of time. Play one game of DoTA and then tell me "Team wide voice chat isn't a good idea". The amount of coordination that you get from having voice comms is astronomical. Night and day.
I think you would be hard pressed to come up with a situation where muting someones voice comms in game wouldn't be sufficient in dealing with someone being toxic.
source on article: https://nexus.leagueoflegends.com/en-us/2017/12/dev-upcoming-party-changes/