Our systems are touchscreens. Sometimes 2-4 of them. We go many hours, some more than 12-14 at a time, programming with touchscreens, in addition to hard surface controls. Half an hour? Ha. Somehow, our arms are not in pain doing this day after day....
That is because most of the time you are using the horizontal controls. Imagine if that control board with all the sliders, faders, etc were on a vertical panel on front of you. THAT is how using a touch screen PC would be for that functionality without the specialized hardware -- hardware that is necessary because of how non-ergonomic sustained use of vertical control surfaces are.
It wouldn't make sense for Apple to alter their entire UI, hardware and API infrastructure for little niche markets like this.
It's a lot more than making a "touch iMac". The hardware is nothing without the software, which includes touch-aware OS, touch-aware APIs, touch-aware programming guidelines, touch-aware developer training, etc. Touch-aware apps have a significant UI mismatch to non-touch apps. E.g, you can't just "port" full-featured Photoshop to a touch platform. Where do you put the 100s of menu options, tabbed style sheets, etc? The UI objects (scroll bars, check boxes, etc) are not sized for touch selection nor is the OS and app logic designed to receive and process touch events. The entire app must be redesigned.
...If market research shows that almost no one uses it, then why are there so many PC touchscreen all-in-ones being made? Does only Apple understand the market...
Those PC touchscreens are running Windows 8 or later and the Metro/Modern UI. After so many failed iterations at mobile platforms, and seeing Apple pass them by in that space, Microsoft frantically tried to use their desktop strength to force-feed a touch UI down customer's throats -- the Metro/Modern UI and underlying API system. It was poorly executed and damaged the company. Most key Microsoft executives and program managers in charge of it were either fired, demoted or resigned.
The Win8 and later touch UI works in two modes: for Metro/Modern apps they have the infrastructure to process touch events directly. However the UI and control constructs are simple and limited relative to a desktop app. A Metro/Modern app does not have access to the rich array of APIs (memory, threading, synchronization, etc) that OS X or Windows desktop apps have. Therefore they are best suited for lightweight apps. I doubt you will ever see a full-featured version of Photoshop or Premiere Pro using *either* Microsoft's Metro/Modern UI or Apple's iOS -- at least in their current form.
The forces a split in the development pathways, OS and API support. Apple has chosen to split that along hardware lines, with iOS and OS X running on unique hardware. Microsoft is trying to merge Metro/Modern and Windows desktop onto a single platform.
On touch-enabled hardware, Windows 8 and later can remap touch events to keyboard/mouse events in a desktop app. After a fashion you can interact with those via touch even though the apps were not designed for that -- but it's very clunky. It's akin to using a web site designed for a PC from an iPhone, but worse.
A Windows Metro/Modern app on a touch PC might actually be a good fit for the scenario you showed -- assuming all the other support hardware is there. If the app need not be highly complex and just used to select modes, memory banks, EQ settings, etc, that is possible. However if you hit the complexity/capability limit of what a Metro/Modern app can do (from either a UI standpoint or underlying API) then it would have to be a Win32 desktop app and use the Windows touch remapping to fool the app into thinking it's receiving mouse/keyboard events. That is usually not a good choice either.
So the reasons why Apple thus far has not pursued a touch-oriented OS X are quite complex, but well-grounded. Ergonomics are only one aspect. This isn't to say Apple will stay on this course forever, but Microsoft's disastrous foray indicates a need for caution.