Not OP, but Ballmer's attempt to reinvent Windows Mobile to compete with modern smartphones was too little too late.
Microsoft during that era was one bad call after another. When Apple saw multitouch they made a phone; Microsoft made a giant table[0] (very little imagination beyond the Jeff Han demo[1]). When Microsoft saw the iPhone reception, they sunk 2 years an $1B into the Kin line of phones[2], which lasted all of 2 months before being pulled from shelves. It was several years later when they bought Nokia, when then iPhone was already 6-7 generations in. By this time the world was already moving to "mobile first", and Microsoft was being left behind without a platform.
For a couple decades Microsoft worked to convince the world that Windows, Office, and Microsoft as a whole was needed to get real work done. It was during Ballmer's time at the helm that this belief eroded. People saw they didn't need Microsoft to make a good smart phone. They found they could get work done with Google Docs and didn't always need MS Office. They found macOS could be used to get work done just as well as Windows... and in fact, a desktop OS might not be needed at all. Ballmer's biggest failure was giving his competitors time to show the world that Microsoft wasn't as necessary as the world was led to believe.
As far as Teams goes... it is used a lot by companies that are all-in on O365, but Slack and Zoom were the two household names in the space during the pandemic. Once again, the world was shown that work could be done without Microsoft.
[0] https://en.wikipedia.org/wiki/Microsoft_PixelSense
[1] https://www.ted.com/talks/jeff_han_the_radical_promise_of_th...
[2] https://en.wikipedia.org/wiki/Microsoft_Kin