"I hate to say this, but the Microsoft Teams product is better than the Slack offering. And to be honest, if an organisation is paying for an Office 365 license anyway, why would they want to use Slack for another cost?"
That's the whole point of determining the behaviour to be anticompetitive.
If teams *is* better, then they should be able to charge for it as a standalone product (or at least have it as an optional extra on the subscription - there is no need to not have the possible benefits of integration).
I find Slack does very well for my use case, I have a couple of integrations which mean that most of my day is spent with slack on one monitor, telling me about updates to tickets, booked meetings etc...
I virtually never log into email or calendar apps, because I don't need to. I'm always logged into to the ticketing system, but slack gives me a good feed of when there are tickets I need to check back on - without being distracting of the work I am doing on another ticket at the time.