I don't really see anything compelling here yet. "Team" focused platforms heavily leverage document sharing/hosting/editing/etc. It's going to be very hard for Discourse to make headway there I think. I don't see calendars & asynchronous chat, polls, etc enticing anybody to switch.
Yeah, for chat Slack and MS Teams are the go to, and they're free to start with. I think Discourse will have a tough time not having any kind of free starter tier. But I do like the idea in general of having proper forum threads with a full editor that can be organised into nodes to discuss projects with team members.
It seems JetBrains have just released their own Teams platform called Space too.