You seriously can't come up with a better way to solve this problem than replacing a very efficient, very robust light source with a very inefficient, very delicate one, secure in the knowledge that as it runs all year round, some tiny fraction of the huge amount of waste heat it generates will go towards melting snow?
Although I'll note in your defense that "incandescent lightbulbs" are more accurately described as "heat lamps", and one potential solution might well involve a specialized heat lamp which only activates when the light itself is obstructed.
Or some kind of centralized switch.
Totally doable of course, but it does make them more complicated, more expensive and less reliable.
You would need to calculate the tradeofs. It's not automatic either way.
Apropos of nothing, but this just made me look up "defroster" on Wikipedia... "For primary defogging, heat is generally provided by the vehicle's engine coolant via the heater core; fresh air is blown through the heater core and then ducted to and distributed over the interior surface of the windshield by a blower. This air is in many cases first dehumidified by passing it through the vehicle's operating air conditioning evaporator." Wikipedia makes everything so cool.
