Why aren't plants black? 🌿

I think you missed one theory: plants are green because the sky is blue. How does that make sense? Sunsets are red for the same reason the sky is blue. As you point out there is no evolutionary advantage in absorbing more light during most daylight hours, since saturation has occurred. Indeed there is disadvantage in absorbing light that cannot be used, since it can cause the plant to overheat. However at the ends of the day, dawn and dusk, there is evolutionary advantage in being able to absorb more light, since saturation is not occurring at that time. So plants that can efficiently absorb the red light of dawn and dusk and can reflect away unnecessary light during the main part of the day have the evolutionary advantage. Green leaves reflect green light and absorb red light.

So plants are green because sunsets are red. Or, alternatively, and slightly more obscurely, plants are green because the sky is blue.