Yes , exposure to air/orygen is the main culprit in decay of Long Term food supplies .
Vacuum seal removes the oxygen , but even 'seal a meal' plastics will leak over a long period of time ;
but is rarely noticed in freezing (which inhibits bacterial growth) since those foods a more frequently consumed within weeks of being put in storage.
That is why even mylar sealed bags encourage the use of 'oxygen absorbers' for long term food supplies usage .
Volatile oils such as olive oils turn 'musty' even though there isn't much air in the bottle ; storage of olive oil should be rotated less than 18 months of pressing,
and there isn't that much room for air in the bottle in the first place.
Fat , a more liquid form of volatile oils , can turn musty and rancid within weeks , under the wrong storage conditions, and contaminate the food product.
That is why we are encouraged to rotate even canned good supplies on a regular and timely basis (ie: botulism ).
Can the mylar bags be sealed with a vacuum packer, or will that melt them? I usually see people sealing them with an iron, but getting air out seems awkward with the mylar.
-JT