"I have literally considered, several times, my "ideal startup" if I were a millionaire and needed to run something to keep me occupied. One of the first rules would be "No Windows or Mac" "
And that's possible for a start-up, with some pain and significantly limited choice. Many technology startups start with Macs funnily enough. That doesn't usually last long though.
"I lived on a Linux desktop for many, many years, even managing Windows networks from it."
Because that doesn't require a suite of typical business applications - and obviously you still had to RDP to a Windows box to run management tools - so basically you ran a VDI Windows client!
"Are you seriously telling me that Munich couldn't go down that route"
You seriously think in over a decade they haven't tried their best? According to Munich's IT department on top of the project costs, they have spent €82 million! on remediating products to work with their current setup! See:
https://www.techrepublic.com/article/after-three-years-of-linux-munich-reveals-draft-of-crunch-report-that-could-decide-its-open-source/
"so you literally just get a VMWare window, inside the Linux desktop, of whatever fancy-schmancy app it is that absolutely can't run on anything else?"
Well Citrix is usually the VDI choice of enterprises, but yes they did do that. When people needed a version of Office that actually worked it was via VDI to a Windows box.
And if you are going to do that - why bother with a fat Linux client on the desktop? - just virtualise everything. Where I work we have zero desktops - even in IT - everything is via VDI and Wyse terminals. And FYI the higher end clients like the top of the range Wyse 7040 only run Windows Embedded due to it's better security, features, performance and remote management capabilities than Linux - which is only used on low end boxes. If you don't believe that's why then go read the product descriptions!
"Hell, you could do it via a cloud service, even."
Well, yes. Lots of people are looking at cloud desktop and VDI services on Azure and AWS. Not on Linux though!