PET base long term stability of at least 500 years has been proved by the Rochester Institute of Technology.
The YCM Separation Archival Master the way A.M.P.A.S define it is black-and-white polyester film stock.
This kind of testing, is called accelerated age testing, it could be right, it could be wrong, the only for sure way to tell, is to take a piece, let it sit for 500 years, then look at it. Realistically though, beyond my (potential) grandchildren, few people will care about my work, so if my images last another 100 years or so, that's good enough.... I have some 40 year old B&W acetate images, that are like new, so and like I said, an image of my grandparents is ~65 years old, and although it's grainy and the focus isn't great it was probably like that when new, they were farmers, so simple box cameras would have been the rule.