But for the rest, I doubt MS would have succeeded in those markets even under the best of circumstances. The iPod made Apple big again, but without Steve Jobs as the pitchman, it wouldn't have had the massive impact it did. The offerings of MS (the Zune) and others (Creative, Sandisk, Toshiba, Sony, etc.) were more than good enough, and they worked with a variety of online music stores. It's just without the brand cachet of the iPod and Apple's lifestyle marketing, it's hard to get excited about an MP3 player.
Slack and Dropbox are both huge inside startups, but nonexistent in old-school enterprise companies. Nobody at work wants yet another login to have to remember. I know this because I work inside a digital team at a 3000-person company where everyone swears by O365's out of the box capabilities. Skype for Business and OneDrive for Business inside of an O365 plan is "good enough" for a lot of people.
Dropbox has made losses each year since 2015. It's great that it's achieved good market share since it started in 2007, but maybe there's a limited enterprise market for something that's a one-trick pony. And one-trick ponies are the polar opposite of MS' strategy.