One thing bureaucracies are susceptible to is vendors requiring mandatory software updates on custom software. This costs much more than most people think, and with the city of City of Munich would dwarf any productivity overhead.

Sure it's the case with Photoshop as it's an off the shelf piece product, but it won't be with CustomVendorDirectXAccounting v13.5. I doubt City of Munich has teams of people running photoshop, and any designers likely have suite of macs anyway.

I'd wager the cost savings of switching to OpenSource likely won't come from initial costs. It will come from forcing custom software vendors to distribute software that inherently requires less ongoing costs.