...there are two factors working against an 
overall decline in their value.

1) Collectors are taking better and better care of 
their collections.  This should make cels last 
even longer.

2) Most importantly, many animation companies are 
moving away from cels and into computer animation. 
 Cels will soon become largely extinct.

Even a rotten tulip bulb would be worth someting 
if healthy tulips no longer existed.  ;-)

JP