Slack is the new Sharepoint, everyone uses it but no one really wants to and it doesn't really fit the bill but everyone else is doing it so companies adopt it.
Microsoft Teams Chat UI is garbage. It uses a lot of space, i can't just have a list of channels, no i need groups etc.
It even doesn't matter to me if its slack or hipchat or matrix or whatever. I don't know why we migrated to slack.
I don't get why Teams doesn't provide a lightweight chat typical UI. I would just switch to Teams. After all video calls are much better on Teams and more stable.
But then Slack integrated Teams and Zoom. They clearly don't want to compete in this area and i also don't get why.
And yes, Atlassian did drop the ball. We'll see the fallout in five years, I don't see how they can save the company now.
Growing at that rate at this point in your company's lifecycle is pretty extraordinary. So not sure why you would consider them to be a cautionary tale.
As for what has happened to them. They have been on an acquisition binge the last few years e.g. Trello, StatusPage, OpsGenie, Mindville etc. And as mentioned above they have been rewriting their UI in a new React framework called Atlaskit.
I also used to think Atlassian was a pretty poorly run company but then you look into it and actually the opposite is true. Although they did let Github, Gitlab, Slack dominate them.
Next few years will be interesting as Gitlab is moving very fast into their space.