Many arguments can be made for and against touchscreen Macs -- personally, although I think touch functionality can be good sometimes, based on past experience with 2-in-1 PC laptops an OS not optimised for touchscreen input (such as macOS or Windows) ends up making the feature frustrating and not particularly useful...
The "affordances" of using a touch screen or stylus are significantly different from a pointer-based interface. From obvious stuff like needing bigger buttons and icons, to how you select regions of text, how you implement the equivalent of things like ctrl-click/right click,, shift click, double click, pointer entering/leaving region, hover... or, going the other way, multi-touch events. None of it is rocket surgery, but it doesn't fix itself. Then there's the whole "gorilla arms" issue of the screen on a desktop or clamshell laptop just being in the wrong place...
Part of the genius of the iPhone was
completely committing to a touch interface with minimal physical buttons and requiring all of the apps to be specifically designed for that - c.f. contemporary Windows phones that often had touch screens, toothpick styluses, keypads, various function buttons
and scroll wheels, joypads and even slide-out QWERTY keyboards (yup, I had a phone with the complete set...) - and software that was carefully not optimised for any one of them.
So a Mac touch screen would always be an optional extra - there are a few cases where tapping the screen and multitouch would just be more natural (mixer sliders and on-screen piano keyboards in Logic would be one example, or for a stupid example,
Plants vs. Zombies was
so much better on an iPad...) - and other cases where using a stylus for drawing and writing makes sense.
One pro-touch argument would be that, now that the Mac can run iPhone/iPad software natively, some such apps would be basically unusable without touch (possibly one of many reasons why so few developers have enabled MacOS use of their apps). That wouldn't solve the lack of GPS, accelerometer, rear camera etc., though. Personally, the
main time I've felt the need for a touchscreen is testing whether web apps were usable on Windows laptops - I don't think that's a mainstream need, though.
I don't think there would be a lot of point simply slapping a touch screen on an existing Mac - it would be a case of coming up with some sort of "convertible" design with desktop/tablet/easel modes. I've always loved the MS Surface Studio concept (either as an all-in-one or standalone display) - pity the cost vs. specs of the actual hardware makes Apple look like an economy brand.