I believe there are some strong anti-trust arguments for it to be possible to pay for only the OS you want to buy, not the one the manufacturer bundles with your device?

Granted there's still a de-facto monopoly on paid for desktop operating systems for general x86 pcs -- but does that really mean that we should try to keep it that way?

That's not how systems are designed and built. You're not paying for the OS, you're buying an integrated product that wouldn't exist unless it was designed and manufactured for Windows.