Acetate slides can degrade after 40+ years, and convert back to vinegar,
but polyester slides will be around for centuries 1000+ years.
Polyester as a film base, has not been around long enough to know how long it lasts, it may last 1000 years, it may only last 20, nobody really knows. Funny thing is, I have an old B&W negative of my Grandparents, it's safety film, so acetate, it's also old enough to collect an old age pension. It has not been well stored, but acetate can last a lot longer then 40 years, if stored properly, and sometimes not so properly. Heck there are still millions of images on nitrate film, still in excellent condition, even though it hasn't been used since the 1940's, for still use.