... and nearly all European corporations run US-made operating systems on some of their machines, many of which are critically important.

A real untangling of the US and European economies seem both impractical and really inefficient.

I feel like this overstates the importance a bit. Sure macOS and Windows can claim to be involved in a great many things but no computer system running Windows is that important and couldn't be replaced given you have quite a large amount of lead time before the software is inoperable.

I've worked with airgapped Windows XP machines still in operation running scientific instruments.

If you deleted Microsoft and Apple from world it world it would be a Y2K event but the world wouldn't end, it would just be a lot of work.

At this point it's inevitable, no matter the cost.