I would rather have those shortcuts visible on the screen, where I am already looking, rather than having to peer down at the keyboard to find them. And I'd also rather use the methods I already have for issuing commands to my computer, the keyboard and pointing device, rather than using a third method for some random subset of commands.

than are practical to memorize? How many could you fit on the touchbar? 20?

Around 60 I think. About six that were perpetually visible, then a dozen that were revealed via a "folder" that appeared when one of those was tapped. The remainder swapped out per-app (like, one set for Safari, another for XCode, etc.).