> where it's a super-useful ultra-flexible slider/picker that works far better than a trackpad or mouse for that purpose.

You know what else does that? A touch screen. Apple seems to be okay with it for the phones/pads. It would also be cheaper (since it's just an overlay on the glass). And the touch area would be the size of the screen that you're looking at, rather than a narrow strip you have to shift your eyes to.