I'm not familiar with Office licensing. Do you have to pay more because they bundled Teams (maybe) or do you have to use it? (that would be surprising)
Anyway, at least you can self host Exchange. Slack is centralized.
We had an O365 plan that included Skype for Business. Skype was replaced by Teams. MS force 'upgraded' our O365 tenant to Teams - I was given notification but no option to completely avoid Teams. We don't pay extra specifically for Teams, we do pay extra for PSTN dial-in to Teams meetings.
No, we don't have to use Teams. But we were using Skype, so we were going to move to something. No one was already using Slack and paying for just chat wasn't going to be in the budget. We can certainly uninstall it. But you have to be sure to uninstall the machine-wide installer they stick on there or it comes back.
After covid, we hold most meetings in Teams now and its been fine. Wish I had more control at the beginning and it didn't just show up where we already had Skype.
Right we were using Skype. Then one day I got an email about Microsoft moving us to Teams. I was able to delay the Teams roll out by 1 month but I could not prevent us being moved to Teams. Skype was part of the Office 365 level that we subscribe to and we were using it. MS replaced it with Teams.
Honestly, Teams is better than Skype. But it wasn't until covid forced everyone home that we really started using Teams, that first week was painful.
I was fortunate in that I moved my team to Teams (I still find the name terrible) well before MS put a mandate out.
So our folks were comfortable once the MS EOL statement came out and the company started moving everyone over. Everyone else had quite a bit of a struggle for a week or so though.
On the plus side, I think there were very few complaints after a couple of weeks. Which probably had a lot more to do with how bad Skype for Business was (much worse than Lync) than how good Teams was (it was still fairly unpolished at the time...it's much better now...MS has been adding features and fixing issues rapidly).
Anyway, at least you can self host Exchange. Slack is centralized.