...I think my production cels are probably more 
acidic than the fancels I own. ^_^

The plasticizers in plastic are what gives 
plastic its bendability. Plasticizers are also 
what gives plastic much of its acidity. The more 
bendability a plastic sheet has, the more 
plasticizers an item has, and my production cels 
most definitely are much more flexible than the 
fancels I have. As animation is already an 
expensive process, I sincerely doubt studios care 
enough to use archival materials, especially as, 
for their purposes, the image only has to last 
long enough to be filmed. 

In acid-free plastics, like acetate, they replace 
those plasticizers with "other" things, so it 
remains pliant, but it has a generally more 
brittle feel to it. There are some products, 
though, that while they don't lay claim to being 
acid-free, they do lay claim to 
being "dimensionally stable" and "non-yellowing", 
and those are always worth experimenting with.

But in general, I think if someone treats their 
fancel like art should be treated--- ie, keeping 
it out of sunlight, keeping it out of heat, and 
so on--- it should have a decent lifetime. I do 
keep an eye to try and see how long an expected 
lifetime is going to be, and so far, I've passed 
the three-year mark with no visible deterioration 
in lines, paint, or acetate. ^_^