A manifesto on shower temperature control

Adding proportional control isn't too hard. Certainly an easy option would be actuating the valve electronically and just using an encoder in the knob to control it digitally. There are a lot of advantages to this, such as being able to automatically comp for actual water temperatures and customizing settings for personal preference, but it's understandable that people might feel uneasy about a cheap electronic device in close proximity to the water they are in. I'd point out that this hasn't stopped us from putting things like lights in our showers which by necessity draw much more power, but some may still be concerned. More likely it is the lack of experience with electronics that valve manufacturers and plumbers have that make them hesitant to change.

For a mechanical solution, you could transmit the torque via a gearbox instead of a direct shaft. Switch gears to get higher or lower resolution control. You can even have a single set of gears with variable diameter sections to have higher precision in a fixed subsection of the range. Unfortunately this would not allow for easy adjustment for an individual installation, nonetheless different people using the same shower. The increase in manufacturing cost and installation complexity would probably not be huge, but in such a high competition commodity market it would be noticeable.

But solving the real issue, a lack of fine control in the important range, is actually much easier to solve. Just make the handle on the knob longer/larger diameter. Especially if you just want to modify an existing installation, attaching an 18 inch handle onto the existing knob is something just about anyone could do in a few minutes. For precise control, grip it on the outside, for fast large changes, choke up on it. For the OEM, a design with a long handle is pennies more expensive to produce at scale, easily justified for even a very small competitive advantage. Maybe some people really like the aesthetics of a small handle, but I've never once heard someone complain a shower handle was too big. I still think the electronic controller option is better, as you can deal with other shower temperature control problems like the long heat up times of the pipes and the variable ideal water temperature at different points in the shower, but the perfect can be the enemy of the good enough. Frankly it's just weird that I can't seem to find a shower handle longer than about 6 inches available for sale right now in a quick google search.

I can testify to that because in our house it works that way. The only time a shower is affected is when filling the large bathtub - which fills much faster than in most homes because it has a very large feed but that's the tradeoff since there isn't enough volume from the utility in that one case. Otherwise people can flush toilets, use the kitchen sink, wash clothes, etc and showers are not affected whatsoever.

Another factor is insulation - even many modern homes don't insulate the hot water pipes so even with an instant hot water heater that responds quickly you still have to pay the lost heat cost of warming the pipes and everything the pipes touch before the temperature fully stabilizes. This is a cost-saving measure - there is no reason the pipes couldn't be insulated. If you throw in a recirc pump then you can have instant hot water at every tap with very low energy cost.