CNET spoke with Craig Federighi after last week's keynote, and one of the questions they ask him is whether there will be a touchscreen Mac (around 2:30 in the video):
Craig Federighi: At Apple we build prototypes around all sorts of ideas. So we certainly explored the topic deeply many years ago and had working models, but we decided it really was a compromise. For a device you hold in your hand like a phone or tablet it is very natural to rest your hand on the tablet and work that way. We think touch is at its best and we wanted to build, and have built, a really deep experience around a multi-touch first user interface. Grafting touch onto something that was fundamentally designed around a precise pointer really compromises the experience.
Those were carefully chosen words by Federighi. He does not say that there won't be a touchscreen Mac, instead he notes that the simple addition or "grafting" on of a touchscreen to the Mac would be a compromise. Importantly, the compromise that he refers to is not one related to ergonomics, but rather the fact that macOS is currently designed around an interaction model driven by a precise pointer.
I agree with Federighi. I certainly wouldn't want to see a Mac with a touchscreen bolted on, with no adjustments to the UI of macOS. But as someone who regularly uses the iPad Pro in a laptop-esque configuration with the Smart Keyboard, I see the value in having a touchscreen on a Mac, provided that there are also UI changes to macOS. I don't expect this any time soon, but I do think it will happen.