The new look implemented too much white colour across the OS. This in turn made it too bright and people asked for dark mode.
Ideally, there's no need for dark mode if UI is balanced. If it implements all kinds of colours.
Examples:
1. Stacks layout was black up until Yosemite.
It was changed into white.
Same goes with Finder. Too much white, too bight.
OS X Snow Leopard for comparison. It has grey header, light blue toolbar, almost no bright white colour. It's easier on the eyes and there's no need for dark mode.
Same goes with iMessage
Monterey:
Mountain Lion: