Agianst my better judgment I’m going to wade into the swamp. Why are “we” better off not having dual boot option. “You” may feel you are better off for it because it doesn’t spoil your pure iPad world, but for those users willing to accept the compromises of poor touch capabilities for having an ultra-modern, ultra-portable MacBook when we need it, why shouldn’t we get that? Why should Abazigal and ericwn determine what me and exoticSpice have to live with? Let ME make that choice, not you (or Apple)...
The TL;DR is that it would run counter to the very essence of what makes an iPad an iPad, and it would run counter to the very essence of what makes Apple uniquely Apple. The day Apple goes down that road is the day Apple stops becoming a design company and turns into pretty much any other tech company out there.
It's the same reason I don't go insisting that my local Japanese restaurant start carrying French cuisine, reasoning that "diners don't have to order it if they don't like it". When I go into a Japanese restaurant, I expect a certain experience. I accept that the food and ambience is done a certain way for a certain reason, along with their decision to not serve certain types of food that doesn't match the theme of the restaurant.
To me, Apple has always been more than the products they sell. I like Apple because in my eyes, the company represents a belief and an ideal - that design needs to be the guiding element throughout a product's development timeline. It used to be be that design simply wasn’t important in the tech industry. The industry’s leaders created crappy software and crappy hardware, and PCs were these uninspiring beige boxes. Apple’s success totally upended the industry’s value system, and showed that design can matter in the mass market, where the end user is the customer and gets to vote with their wallet.
Almost all of Apple’s competitors value design more today than they did a decade ago: Microsoft, Google, Samsung — all of them, and we are (mostly) better off for it.
IMO, design is what sets Apple apart from the competition. Allowing an iPad to run iPadOS may work from a purely engineering perspective (and I agree that there really is little technical reason why it shouldn't work), but Apple isn't an engineering-led company, and I feel there are some things they simply shouldn't do, because it goes against the basic tenets of design, where as Apple designers call the shots, and search for and having technology made to serve the product experience, not engineers excited about about new hot tech and trying to turn it into a product.