It would obviously add cost, how much is debatable, however the major issue would if the form factor of the MacBook would change to allow the device to be operated in tablet mode.
I have a Microsoft Surface Laptop (to keep me familiar with the Windows world) and I can safely say I have never ever used the touch screen on the device. Mixing keyboard, touch pad and, maybe, mouse input devices with touch screen, does not work for me. However I can see that dual purpose laptop/tablet mode might make sense like the Microsoft Surface ( not the laptop version). Apple do not have such a device and MacOS is not a touch or stylus friendly OS. Thus a lot of changes would have taken place place in addition to adding a touch sensitive screen. Note that in the Windows world a touch sensitive screen can add 10 to 15% to the price.