You've deviated from the original point of your GGGP post. My reply is that you should use tools optimized for the tasks you perform, and with computers, the operating system's HCI model is very much a functional part of the system. Screwdrivers can be used to hammer in nails, and hammers can drive in screws (very poorly), but it's better to use the appropriate tools for the job. You shouldn't expect to use the tools and skills optimized for content consumption to carry over for content creation. Portions of the hardware may be general purpose (although even that's debatable as mobile processors are optimized for low power consumption whereas desktop components don't have the same power constraints and tend to be optimized for performance), but the O/S HCI shouldn't be. You don't use a shovel to dig a swimming pool, and a backhoe is overkill for tilling a vegetable garden.
But the difference between the UI of a tablet and the UI of a desktop PC is a matter of software, not hardware.
So touchscreens are a common and primary form of input for desktops? Never mind. However I see your (new) point regarding adding a keyboard and mouse to a tablet (presumably via Bluetooth, although I suppose you might be able to use a powered USB hub with some tablets). Yes, Windows is proprietary system created by a for-profit corporation which chooses to enforce market segmentation to maximize profits. They've been doing it, and been publicly lambasted for it for at least 20 years, so it shouldn't come as too much of a surprise. If you want to use your tablet as a desktop for content creation, you could always wipe Windows 8.1 from your tablet and switch to a Linux distribution, or perhaps re-size/re-partition storage and dual boot. Ideally Windows would present a Metro interface in tablet mode, and detect when you are plugged in to a keyboard/mouse/screen dock and automatically switch to a desktop UI. Microsoft have chosen to not do what is best for their users (provide flexibility) to make up for the fact that they are years behind competing mobile O/S solutions in developing mobile apps by artificially pumping up the market for Metro apps for developers. Caveat Emptor.
The analogy works very well. Shovels and backhoes have very different user interfaces, even though they both shovel dirt. But you use them for qualitatively different things. You use the shovel to do some gardening in your backyard. But if you decide to put in a swimming pool in, you're quickly going to find out that the shovel isn't going to cut it. And if you realize you need a backhoe, you're either going to need someone skilled to drive it, or you're going to need to pick up a whole new set of skills to control it than the ones you learned for gardening with the shovel.
Tablets, smartphones, and desktops all are general purpose computers that process electronic data. Tablets and phones are devices primarily optimized for media/content consumption. In terms of functional complexity and data entry volume, that is a qualitatively different task than that of the typical uses of content creation/manipulation for business desktops/users. The simplified UI that is a boon for the former, is a dragging anchor for the latter.
How is it then that we can afford so much better medical services, so much better air transportation, so much better everything else, but if we want reliable code its going to cost extreme amounts of money?
Most medical practice is based on a limited set of diseases. It's a large set, but it's not the medical equivalent of Turing complete. When patients get sick and no known treatment is known for that disease, they sometimes die. Sometimes the treatments are only partially effective. Many times, patients are given the wrong drugs or doses in hospitals with harmful or even deadly results. Patients get mixed up and have the wrong operations performed on them (to the point that it's getting more common for the area scheduled to be operated on to get marked up with ink while the patient is still conscious). Such are the bugs in the medical field that many lawyers get rich from litigation over those bugs.