What material are those bags made of? If it's not at least polyester, all the speculation about nitrogen is futile. Oxygen permeability varies greatly from one plastic to another.
When Eastman Kodak was about to declare bankruptcy, after several email exchanges with Dick Dickerson and Silvia Zawadzki (the inventors), I bought what for me was a lifetime supply of XTOL. At the time, most Kodak powdered chemicals, including XTOL, were packaged in aluminized polyester packets. I then further placed those in zip-closed aluminized polyester anti-static storage bags, the latter not for their anti-static properties but rather as a secondary oxygen barrier. Dick and Silvia said they couldn't see any reason why my approach wouldn't work. Nearly a decade later, when I mix another five liters of stock solution every six months or so, there's no discernible change in the powders or resulting developer.
If anyone is concerned about long-term storage of powdered chemicals, I strongly recommend placing manufacturers' packages into zip-top reclosable aluminized polyester anti-static bags.