Would a wireless volume knob for home theaters be worth it?

No, a knob would not be worth it. One of those big handles like in an airplane cockpit, that's the way to go.

I cannot imagine big knob on my remote control, so perhaps either small knob or a slider.  Buttons on my remote have just right authority/speed and anything else would be just strange (unnecessary).  On the other hand I enjoy knob on my DAC/Preamp and I wouldn't like buttons instead.

At the risk of sounding like an advertisement (I'm trying to find out how many people might be interested in this idea) take a look at this, imagine this being your stereo remote control:

https://i.imgur.com/rncgnDm.jpg

For the most part I think manufacturers have licked the issue of speed of change to up/down buttons. That is, they seem to work well enough for me.

So what you're saying, they have licked the knob.