Yeah, cars wear out and value go down in the long run.
Most cels' values do usually go up, but the cels wear out in 
the long run too, depending how you take the special 
care on them...but most collectors don't preserve them 
in the correct ways.
For example, ink lines fade out, acetetes decay and
create the white dusty-liked dots between cel layers, 
and the pencil drawings turn yellowish...etc.