I agree that we should be able to set lighting by physical units rather than by a dimensionless number.
Watts are a measure of the amount of electricity consumed by an electric light source, but the amount of light emitted per watt varies from one device to another, depending on its efficiency, so watts are not a good way of setting lighting values. Dimmable lights make them even less desirable.
Lumens are a measure of the amount of light emitted and therefore allow direct comparison of one light source to another. Lumen levels are commonly provided by lighting manufacturers. Lumens are used in lighting calculations to derive Lux, the amount of light per unit area which is used to define task-based lighting requirements. Please make Lumens the measure of light emission!