It's the City of Munich. They don't have a CRM. They don't have customers, they have subjects
And Exchange - how many of these people have a packed agenda that they need something like Outlook to shuffle around appointments?
GIS-software and other specialized software for all kinds of things (large and small) the city manages and runs is probably a bigger problem. IIRC, they run thousands of pieces of software altogether. Most of that only available on Windows. They could have (and did so, to some degree, AFAIK) run it on Terminal-servers or Citrix - but that probably still incurred significant CAL-costs...
Also, only very recently has Linux started to get "Ok-ish" for mobile use.
That said, Linux on the desktop was never going to happen. It's an oxymoron. To succeed on the desktop, you need something like Apple's or Microsoft's development-models that are at the same time fundamentally opposite to what Linux is, at its core.
Though, some of the people in Munich welcoming this change (employees weren't exactly thrilled about Linux anyway) are going to have a rude awaking when they realize that their Windows 10 client has to be locked down to point where it barely exceeds the capabilities of a thin-client (or a Linux-Desktop) because the threat-landscape has completely changed while they were running Linux and LibreOffice. Back in the day, APTs, crypto-ransom threats existed but were very rare. They're everyday's business now.