Microsoft has, over the years, pointedly remarked that its Teams collaboration environment has many more users than Slack. The subtext to those jibes is that Redmond is the outfit really driving the future of work and Slack is an upstart afterthought.
I use both (paid) Slack and Teams. Teams is a qualitatively worse user experience in almost every way. Other opinions are available of course.
This should count for something, though of course it often doesn't when it comes to workplace software (oh hi Oracle HR, SharePoint).