Not that it'd be easy but I imagine that you can steal their customers by adding chat rooms and integrations slowly to some other type of software, rather than a more direct angle where you have to win at one of those two moments.
But I think your point about MS getting rid of Skype for Business in favor of Teams is the most important one. Once that happens, which MS has stated publicly, Teams will become a necessary app for MS to support, since enterprises won’t tolerate MS not having s chat service in O365.
So my view is that until Microsoft get their act together, Teams is not a serious contender to Slack. Companies can get it "free" together with their Office subscription, but most developers won't use it until it works decently on their development platform of choice.