If the amplitude of the waves coming from a light bulb decreased, which would you expect to happen?
a) the intensity of the light would increase.
b) the light waves would travel faster.
c) the brightness of the bulb would decrease.

The amplitude of the electromagnetic waves coming from
a light bulb is exactly an indication of the energy carried by
the waves. 

If their amplitude decreases, then the waves are carrying less
energy.  The brightness of the light decreases, AND the bulb
runs cooler, because it produces less heat.