You can put Linux on most Surfaces, and many of the cheaper Windows tablets. With pmOS, you may soon be able to put a desktop Linux OS on many cheap Android devices. The UX work for these use cases has been done - GNOME is now very much a mobile-ready and touch-ready environment, supporting real professional work. Plasma Mobile is not too far behind.
If Apple sold a device which was physically identical to an iPad but ran OSX (with some minor affordances to close the gaps between touch, pen, and mouse input- much as Microsoft has pursued in recent Windows iterations) I'd be ecstatic with it. I've considered trying a Modbook, but the price is a bit eye-watering.
Having access to a desktop-style OS would only add to the range of things you could do with smaller devices. Would it be harder to use than an iPad? Maybe.