Ok, so perhaps I overestimated the failure rate just a little.

OK, but let's say you didn't. If bulbs burned out that often, we would
all have a package of spares in the plane. You would check the lights
prior to takeoff, and replace any that failed. If one failed in
flight, then you would go back to the flashlight - which would now be
an emergency, not a normal procedure. As an emergency procedure,
flying by flashlight is fine.

Michael