I don't know how expensive it would be, but I guess price is the reason why there are no synthesizers out there that have motorized faders and potis. Every mid-priced digital mixing console has this in the year 2019 A.D..
Besides layering, it would solve the whole jump-passthrough-relative thing... which annoyes me a little even when working with only one layer. Everytime I want to do a quick jump to a higher value live, I missed the passthrough point and nothing happens. But then the next song I absolutely need the passthrough mode, because I don't want anybody to hear a not-so-smooth transition... So what to do? Knob-mode per preset could be a solution (but that's worth another topic, because a lot of global params would be helpful if they were local parameters).
A Prophet-6-like instrument - which I already classify for myself as a unique premium product - could still be lifted up on a higher level if it had motorized controls.
Are there any negative points about this, that I don't get (Or why does a Moog One not have this at 8000$???)
Considdering most deep synthesizers have hundreds, and even thousands of parameters per preset, I think it's obvious why we do not see any motorised synths... they would be monstrous... otherwise they would have to be created with a set number of motorised faders, which means that you still have to switch between the faders functions, and also in that case have a display that will show what the fader controls... otherwise it would be just as confusing.
This is simply just the tradeoffs you have to make as a developer, if you want a very deep synthesis engine... otherwise you'd end up with less deep engines so that all controls have their own knob.
Personally I actually liked the approach of the P12 module... it did it the most logical way... a single button to enter the relevant display menu for each category of sound parameters, and then a few knobs that give you some live tweakability for the most important performance oriented/minded parameters like Cutoff and resonance.
That coupled with a really well made editor on a computer, and I'm good... it also makes the device less expensive...
With a more simple engine like the P6 or OB-6, I can see the idea of having a knob per function, as this machine is much less complex, and have no layers.
But like with the REV2, I really do not see the need to have all parameters on their own knob (almost, some are in menus still) unless you always use just one layer... as soon as you use Split or Stack the use of the knobs is screwed because they are not encoders, or motorized.
So for me it's simple... it's either one knob per function, or a fully fledged computer editor... anything in between simply will not cut it for me.