> Can they do without Microsoft, however? Can they work without Office, Outlook, Teams and OneDrive?

FOSS to the rescue:

* LibreOffice, instead of MS Office.

* Thunderbird, instead of Outlook (even though I don't like the direction Thunderbird has been going).

* NextCloud, instead of one drive (and there are other alternatives, more FOSS-friendly or less so)

* Matrix/IRC client plus Jitsi, instead of Teams.

and they will do just fine - on Windows or on Linux.