You're suggesting that plastic is ok for long-term aging? Not arguing, I just hear a lot of conventional wisdom to the contrary. If the "O2 permeability of plastic is a thing in long-term storage" belief is just another fallacy, that's good to know.
I was simply suggesting that stainless & glass aren't impervious to oxygen because quite a bit can pass through an airlock.
In my opinion the actual difference between PET and something impermeable like glass or stainless is negligible when you factor in headspace and airlock oxygen transmission.
Several scientific sources suggest that HDPE is vastly more oxygen permeable than PET. They shouldn't really be lumped together when talking about oxygen permeability. Yet, as evidenced by this thread, many people use HDPE vessels for cider, so it's hard to argue.
Anecdotally I've seen numerous wine makers and sour beer brewers use/prefer PET, so I think it's acceptable for most purposes.
My flanders has been sitting in my Fermonster for 7+ months now with no hint of acetic acid, so I'd say it's not excessively oxygen permeable.
As s-met mentions, PET is super common to use for beverage packaging.
Another thing to think about is barrel aging. Lots of wine is barrel aged, right? Oak lets in some oxygen. Corks also let in a controlled amount. It allows the flavors to mature for aged wines.
OP was mainly concerned with cider. Any wine or cider should be sulfited during aging to protect it from oxygen.
Depending on the cider making process, the cider may or may not benefit from aging.
Something to think about.
Cheers