If you use Linux Mint you can replace Office with LibreOffice (offline) and Outlook with one of the many email clients available for Linux.
- One, that probably, for most of the users, what's is locking them to Windows is a bunch of Microsoft applications. Once you've moved out of those apps, you're free to move them to any other OS.
- Two, on government computers there are usually a bunch of things that might be a bit custom and not readily available in other OSes. Smart card readers for weird standards, custom software for security or administration processes, uncommon peripherals...
- Three, there are departments that are going to be tied to Windows for a long time, due to very specific software, for example CAD or things like that
So I'd say Office is a big and easier win that could allow later allow to move a lot of workstations to Linux. Then, you can focus on the other less common uses of Windows and see how feasible is migrating them. But starting directly moving to Linux would mean starting directly with edge cases and problems.
Also Linux Mint isn't community supported. Large corporations and governments like to have operating systems with long support cycles. That why RHEL has like 10 year support cycles.