Perhaps someone should invent a mechanism for changing the tuning of a string by a specific and exact amount, similar to that used on harps. But not necessarily pedal-operated.

Such a creature does exist. (CLICK)

It's a feature of the self-tuning mechanism which is available on selected Gibson guitars. One can pre-program several tunings and the guitar will tune itself to any of them at the push of a button. Problem is it's only available on a few electric models, no acoustics.

Personally, I prefer the multiple-guitar system. Any lame excuse logical reason to own another guitar should be cultivated, not ignored.