I posit that a fretless violin is more akin to a physical keypad than any traditional touch UI. I'm not a violinist, but I'd imagine these physical subtleties would take the form of neck taper, neck thickness, string pitch, string tension, etc. There's an equivalent feedback loop that stern advocates of the physical keypad simply can't discard, whereas traditional touch UIs are deadly efficient at "quantizing" such physical subtleties away.
As an aside, violinists generally don't have a day-to-day multitasking dilemma, say, serenading while driving.
Johnny Lee best explained[1] that the UX pays a cost whenever a UI fails to capture intent...and it's a cost that we often ignore when adopting new tech. Perhaps this cost may be insignicant to the typical low stakes consumer, but to a professional who spends much time corresponding over mobile, retaining a high level of confidence without visual verification or reducing the average number of errors per stroke or quickly adjusting to a new interface based on its tactile nuances is often all the justification that's needed to make the physical keypad an imperative.
[1] http://procrastineering.blogspot.com/2011/07/myth-of-dying-m...
It doesn't compare at all to touchscreen keyboards which have no feedback mechanism at all other than sight.
Violins don't have frets but experts can hit the notes.
I like buttons better too, but there's no need to exaggerate or claim impossibility.