Only PC enthusiasts buy Windows. 99% of the population gets it bundled with their computers and who knows how much MS is charging those OEMs. Probably pennies.

Windows already has a de facto monopoly in desktop OS. They don't need to be nicer and give it for free to get more market share. They have all market share they every will.

It basically already is, at least for consumers. You can download an .iso of whatever the latest Windows version is and install it, and although it will prompt you to put in a product key, nothing stops you from continuing to use it if you don't. You can't customize certain cosmetic settings, and there's a small watermark in the bottom left corner, but it's hard to imagine that it being fully functional otherwise is an oversight rather than something they're fine with. The only people who will go through the effort to install it like that and keep using it are the ones who are least likely to pay for it.

It comes preinstalled on most computers. The consumers don't pay, OEMs do. And they'll continue to pay because most people don't want an OS-less machine.