The only way to prevent dual booting would require a UEFI/BIOS that pulls the OS straight over a network, bypassing local storage entirely.
Even if that didn’t already rule it out, the size that OSes are these days makes it even less likely. At least not unless Microsoft (or whoever) are planning to ditch absolutely everyone who doesn’t have gigabit internet. (It would be kind of funny for an OS to go back to being 1990s-sized to mitigate that though. And funnier still when someone inevitably captures it onto a hard disk anyway.)
A more likely vector would be to deliberately break third party bootloaders every time Windows boots. And that would last until the next anti-trust / monopoly lawsuit and they’d roll it back to the current behaviour of only breaking third party bootloaders on installation.
And even if somehow that didn’t get rolled back, just wait until hardware vendors introduce this thing called a “switch” that can be added just before the power connector on an SSD. Can’t boot from a drive that has no power. BIOS defaults to the next SATA channel. And now you’re booting into Linux.
Doing the same for a mobo-mounted NVMe drive is harder but not impossible.
(FWIW the downvote wasn’t me)
That sounds like you’re suggesting that Microsoft wouldn’t care what was installed locally to be able to net-boot / run the rest of Windows.
I think it’s all but certain that they’d want user’s computers to to boot into something they made, or at the very least, slapped their branding all over, even if that was only a wrapper for their web browser.
I can definitely see them going down the line of saying that their online apps aren’t guaranteed to work under any other system, going so far as to throw in a few deliberate stumbling hazards for anything that isn’t theirs. (Until anti-trust, etc.)
And thus, dual booting will still be something that people do. Even if - as you clarified - they’re not going to cripple that as well.