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.
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.