I have worked in (German) Government, and apart from complacency (and maybe corruption, see Limux) there's nothing stopping the German government (at least at federal level) from adopting open source.

If processes depend on some crappy excel table (created by somebody 20 years ago) or even worse, sharepoint app (commissioned by people who shouldn't be deciding things like this), the processes suck and need to be rebuilt anyhow.

The processes might well be in Microsoft Dynamics 365.

I agree, apart from legal entities because iirc they use some software that's available on windows only