Ive never used anything but slack at work and that's with 4 different organizations. Microsoft teams was tried and ppl just kept using slack regardless.
I'm not saying orgs will switch away from slack. It's a great app. I'm saying orgs that are not currently using slack will not want to pay to switch when they've got a free IM app that works fine. For slack to be worth this valuation they need about 10x as much revenue. I don't see how they could possibly accomplish that when everyone has already picked an app at this point.