If the employees were agnostic about the choice, it’s highly likely that many of these companies’ IT departments would choose Teams instead, because MS is certainly more company friendly (and its bundles tend to be cheaper).
I think Slack may end up regretting losing these non paying customers, who usually end up being their biggest and only cheerleaders in paying contexts.
The other problem for Slack is this decision makes open source alternatives more popular which means more users of the open source alternatives which would likely lead to more maintainers and developers for those projects, leading to them becoming increasingly competitive with Slack. That would squeeze Slack from all sides, with MS hitting them from the enterprise end and these alternatives from the open source end.
It would be difficult for a company to justify paying Slacks high prices when they have all these options instead.