They don't need to design macOS for touch input at all. Just allow touch.
But the interface isn’t optimised for touch. This is a lot more complicated than people often realise and boils down to both conceptual and technical decisions.
When you use an iPhone for instance, you take it for granted than buttons are designed a particular way and act in a certain fashion, but to enable both to work properly and with consistency isn’t just as simple as ‘activating’ a touch surface.
Imagine trying to use the traffic light buttons on a window with a finger. First, they are no where near large enough to be easily pressed (and the scale must accommodate different devices). The software must then know that you intend to press one button over another because of the proximity.
And from a purely conceptual level, Apple would need to market specific devices as having a touchscreen macOS compatibility and not others - and justify those decisions.
Further, the products themselves would need to be completely re-engineered. A MacBook display is not just an iPad display bolted onto a notebook, it’s been designed specifically
not to be touched with pressure and that is precisely why it is so thin and has excellent lamination.
Finally, the trackpad is simply good enough for many tasks that you would use a touchscreen for anyway, from scrolling to zooming to moving windows. The few specific activities that an iPad excels at is enhanced by the fact that the iPad is designed specifically to be held a certain way and used a certain way.
There are things you can't accomplish on the iPad with a keyboard and mouse - but it's fine, and everyone is OK with it because you always have the touch screen there.
This only reiterated my original point; that the keyboard and mouse are
accessories and not integral to the iPad experience. They exist to help users in specific experiences.
There are things you wouldn't be able to accomplish on a Mac with touch - but it's fine, and everyone would be OK with it because you always have the keyboard and trackpad bolted on to the laptop.
Again, this difference being that the Mac hardware would need to be completely re-engineered, marketed, priced and sold.
I’m absolutely not against the concept of a ‘convertible’ and I still maintain that the iPad + Magic Keyboard is a brilliant combination. However, I do believe that if Apple is to pursue a convertible for their most powerful OS in the future, it will be a product line different to the Mac. It would be brand new so as start off with a clean slate for development, not building on top of a legacy platform.