Umm I'm not sure about anyone else here but 
going around and spraying cels with 'fixative' 
probably wouldn't stop the 'cel death' process.  
Alot of cel decay has to do with the elements, 
but I'm fairly sure that the paints themselves 
have a life span.  Remember animation cels 
weren't made for collectors.  They were made to 
last through a quick shot of the camera, and 
that was all.  Cel-Vinyl (the paint they use to 
make most animation cels) wasn't made to produce 
artistic masterpieces so its' durability through 
time probably won't last forever no matter what 
you do.  But maybe someone on here can answer 
you better than I can.  =D