I don't know where you have been operating your monochromatic displays, but
we have some several years old, that are used daily and no sign of burn yet.
Now, of course we are operating instrumentation in an air conditioned
office/research lab, not some hot car.
But, my mobile VHF radios have not suffered screen burn either, in hot
southwestern climate. I try to remember to lower the windows a bit in
summer, but by measurement, I sometimes get to 145 degrees F in the car dash
area and no burn yet.
73, Stuart K5KVH