I'm not sure that they're not just a feature. Even if MS teams sucks compared to Slack, the MS salesman can talk to the CIO of Megacorp and say they'll add it in for half of what Slack costs. They see X million in savings and suddenly the entire organization is using MS Teams instead of Slack.
Dropbox is also one the chopping block to be replaced by OneDrive. Until someone can break hard dependency people have on Office products MS suite will always come out cheaper than Office + $other_thing.
We route alerts through IM primarily, and are remote-first. I get that some folks here complain about their inability to ignore chat notifications, but there's a decent chunk of us that need to find our uninterrupted time where we can, not when we want.
A few weeks ago the startup I work for started using NextCloud as a stop-gap till we get O365 and OneDrive. It is ridiculously simple for us at-least to just copy over our entire Dropbox folder to the NextCloud one.
We have shared channels with Integration Partners, Suppliers, Enterprise Customers... These are extremely valuable ways of communication once setup, especially with simple file-transfer thrown in, etc. Leaving Slack would mean slamming the door in the face of these entities, something we're simply not ready to do for the cost-savings that ditching Slack would mean.
Enterprise users usually just use it for internal messaging.
If you’re big enough for Microsoft to care about you, you already have embedded SEs compensated based on OneDrive and Teams adoption. Your IT middle management pushes teams.
Your CIO is getting the cyber pitch based in the insecurity of your O365 implementation. The fix is to buy Azure AD or the next bundle (EMS), or maybe the E5 O365/Windows subscription.