I’ve found glassine envelopes that are 60 years old and maybe older. They were not acid-free, from that period, though. Others were shredding after 10 or 30 years.
Glassine has a definite grain, so it tears much easier in that direction.
The problems I’ve seen with glassine is in the glue and in the fold. If they pas the PAT, the glue should be no problem, but having a folded flap may leave an indentation over time, which is less likely with stiffer plastics.