This b/w image was achieved by having a black filled layer (white or gray works too!) set to [Color] on top of the original image. Here the values make more sense as the leaves aren't as distinct and the b/w image feels as bright as the colored one.
The desaturated image doesn't feel as bright as the original. More notably the yellow leaves of the Erdtree shouldn't seem to be as dark as the blue sky behind it. It would make more sense for the leaves to be as light as the clouds because yellow is a lighter color than blue.