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.
This is true: my gaming PC had that watermark for nearly 10 years. You can't change the wallpaper, remote desktop doesn't work, but that's the only downside to not paying for windows (and using Microsoft's free iso, instead of pirating a key).
It's quite clear to anyone who's tried it (at least since Win10), that Microsoft does not care at all if you pay for Windows.
