What are you comparing it with?

If you compare Rec.2020 with 10 bits per channel and sRGB at 9.5 bits per channel, both of them using the same peak brightness, you should see similar amounts of banding on both.

Increasing the maximum brightness can require a lot of extra bits to keep things smooth, but that's a separate thing from which colors are possible.

Ah, I see your point, but I still don't understand this argument, because in practice not only we want extra brightness in colours around the usual white points, but also we need the extra potential physical brightness to extend the colour space itself in terms of "colours only" (not sure that this even makes sense ?) because of the differing needs of different colours.

(I'd assume that this is especially relevant for backlit displays like the liquid crystal ones, since they have to use the same potential maximum source power for all of the channels.)

Gamut is affected by brightness but not that much.

Rec.709 has 10 and 12 bit modes too, and it needs them, even though it has the same gamut as sRGB.

The reason 10 bits isn't enough for Rec.2020 is because 10 bits was never enough to fully represent any gamut. Once you add enough bits to properly support a gamut, then extending it to match human vision is a tiny cost.

And to be extra clear, when I initially said "The difference between all visible colors and a more constrained color space", I was talking about taking a color space and expanding its gamut without changing anything else about it.