It did already happen though. It's not just about buttons of course, there is a lot of lobbying, and if other people you are working with (e.g. other municipalities) stay on MS stack, there are going to be compatibility issues that will of course be blamed on the outliers.
The Wikipedia article doesn't make it obvious, but even Munich itself wasn't able to migrate everyone off of the MS stack. One of the linkrotted sources (https://web.archive.org/web/20180516042059/https://www.techr...) says they had about 17% of computers running Windows after the migration "completed" because some required programs couldn't run on their Linux distro. It's not obvious whether the working model was that 17% of staff used Windows exclusively or all staff had to find a Windows box to do some of their work, but either one sounds pretty obnoxious to me.
