If Windows did develop a handheld-specific version, sure, but I very strongly doubt they will and if they did, how good would it be?
Microsoft by now earns WAY more money with its cloud services than with Windows and its toolsets.
And within their cloud services, most servers run what? Linux, so yes MS earns more money with an OS that is not Windows
The part that does run Windows tends to run the "special" server edition - and that actually isn't special at all, inheriting all the flaws from its big brother which is why it is really only used by those devs afraid to learn to use Linux, which is an ever-shrinking part of developers...
So my faith in MS developing a good, non-resource-intensive variant of their OS is pretty damn low.
With Windows, you'll have to make the device more powerful to achieve the same level of power as the Deck for running games, thus increasing price.
I'd get it if the library was very limited on the Deck, but it's just not - almost everything runs fine on Proton - and if not on the default one, there are by now good tools to use other Proton branches on the Deck.
The only games that tend to be reliably problematic are some multiplayer games that go braindead when seeing you're not on Windows and treat you like a cheater
So if someone wanted to use their handheld primarily for that, I'd get it.
Everyone else just gets the downsides of having an OS running on it that was quite simply not made for that kind of usage.
Having to jump to different launchers for games sounds absolutely awful for a handheld experience.