It is this week; how long before it's deprecated in favour of Google's new solution?
That's not much better, it's still not an open standard. Plus since this is a Google chat app both it and it's successor will probably have been killed off in favor of yet another chat app within the next 2 years.
Is there anything that runs on iOS, Android, and Windows? Other than Skype (shudder).
Discord. I’m not sure if Slack can do multi-party video but it has person-to-person.