Running your own desktop is very different from running at scale. Ever managed to get a package conflict, fail to upgrade the kernel, remove some essential software, "curl | sudo sh" something you shouldn't have or break some configuration? Once every year, month or week? With thousands (and thousands) of installs that is a daily occurence.

Paying Microsoft might feel expensive but is in function affordable. What you are paying compared with what goes into, and have gone into, the ecosystem. I'm unlikely to make this argument in a few sentences. The important part is that even with a decrease in scope you would have to pay more for what you want. Or you would have to offer something else. There just isn't the same talent (or resources) available, but you need more talent.

It is questionable whether open source is compatible with sovereignty. There are definitely reservations if it has worked against big tech, so there should be reservations if it works against a superpower. The most mature distribution for enterprise use is likely Red Hat anyway.

This might just be a very big hurdle if it wasn't for the fact that everything is going the opposite direction. There is about zero political, geopolitical, business or popular will to actually do anything that would make a substantial difference. It might actually be illegal to do so since EU rules enforce competition. And the US would object. Which wouldn't be good news for Maersk and Novo Nordisk. (Carlsberg is not big in the US, probably because of US market regulations. Which is a bit ironic).