All the colors in a design are chosen to look good together. Changing one major color could necessitate changing all of them, and likely also icons and image assets. Even photos could be tuned to the contrast ratios of the design.

Changing things that people are used to, like colors, tends to make them upset.

If text used to be black and now it's almost black so something else could be darker, some of the customers are going to be grumpy.