Cels are made to survive long enough to be 
filmed. There's no way to preserve anything 
permanently--- only ways, which you mentioned, to 
slow down the rate of deterioration. But because 
of the materials used and the inherent function 
of the object (short-term use rather than long-
term collectible), they're fragile, temporary 
things. Although it's art, it's a little more 
permanent than refrigerator drawings and a little 
less permanent than an oil painting on canvas. 
^_~ We enjoy them for as long as we can... we 
scan them to share, and to have a record of how 
they once were... but to expect something made 
from such impermanent materials to be in perfect 
condition sixty years from now is a little 
optimistic. The earliest Disney cels, for 
instance, are hand-inked. 101 Dalmations was the 
first show with machine-copied lines, which I 
think is what modern animation studios use on 
cels. So I don't know how those hold up over 
time...

I guess the best thing to do is take care of them 
to the best of our ability, but also keep a dose 
of reality in mind while collecting. :o)

-Cres