That's a double-edged sword, though. Those tax dollars don't just pay for the license, but for ongoing development, responsibility for security issues, support contracts, emergency personnel, and so on. With a purely Open Source strategy, you'll have to pay multiple external consultants to take care of part of this, and/or cover these roles in-house. And suddenly, you've taken up a lot of tasks completely foreign to your business domain, such as new infrastructure and its maintenance, documentation requirements, software development, and so on. And we haven't even talked about the massive effort of educating your entire workforce on new tools and workflows.
Assuming you just replace a proprietary software ecosystem with an Open Source one and immediately get the same thing for free is a very naive view that will get you in trouble.
Having said that, as a German, I am very happy this switch happens and seems to have some backing in the local administration at least. But it's still a high-risk wager and I'm afraid it'll turn out like the LiMux project in Munich, which was eventually (and cleverly so) framed as the origin of all problems in the municipal digital infrastructure. In the end, it got swapped out for a new Microsoft contract in a wonderful example of lobbyism and bribery, and Open Source and Linux have been discredited, to the point no winning mayor candidate can ever bring it up again as a viable alternative.