A sealed MRE doesn't go bad in the traditional way.
Let me back that up. Properly canned food doesn't go bad in the normal way, the modern MRE retort porch is basically a flexible can with an extra outter seal. You can look on youtube for "Boer War Ration" and see a guy in like 2018 eating a ration pack from the goddamn Boer War and its still good because the can -and almsot as important the seal inside the can - was still intact 100+ years later.
"Canning" involves sealing that is either boiled or already boiling in something that will form a moisture and oxygen lock - nothign gets in, nothing gets out. The metal in the can acts as the oxygen barrier; the metal of the can will react with the oxygen oxidizing (that is, rusting) and because metals are usually very oxygen-hungry any oxygen atoms will likely be capture on contact. Inside the can there is some sort of sealant covering the metal forming the moisture barrier.
the retort pouch uses an outter durable seal, a layer of metal foil, and and inner seal. Basically because its encased in plastics, the foil later in the retort pouch doesn't need to be structural just intact for oxygen absorbing purposes.
What will fuck a canned product is two things: bacteria (that managed to survive the canning process orthat enter due to the seal failing), and oxygen entering. Even in a bio-sterile environment the oxygen will cause reactions with the food inside. This will oxide particularly lipids and make them go rancid; if it JUST due to oxygen exposure and not microbial action you can still safely eat rancid food.... you just won't want to because it will taste rancid.
The further back in time you go (or the closer to china/india/africa you go) the more likely you are to find things that were improperly canned - temperature of what goes into the can too low, solid particles read: rodent shit entering the canning process and not getting heated all the way through, etc. This will allow botulism bacterial spores to survive the canning process and go to town in the low-oxygen environment.
The next most likely cause of canned food going bad is failed internal seal. look inside a can and you will notice a thin layer on the metal. This both keeps the metal of the can from leeching into the food, keeps moisture in, and keep out bacteria (but its very thin and easily compromised).
Before the 80s, this would be because of poor quality materials/life of said materials/improper application of those materials.
After the 1980s, its basically doing everything as cheaply as possible and using the minimum amount of sealant material.
the number #1 cause of this seal failing is high temperatures causing it to lose solidity. such as leaving a can in the summer in direct sunlight.
The 3rd most likely is the can being compromised; if we're ignoring catastrophic failure (Read: the can being dropped and damaged) this is usually due to the can rusting through.
We can technically add a 4th if you are doing old, old school canning with glass jars, and that is UV exposure; not an issue with metal cans and retorts. But since I'm being austistically exact might as well cover all the bases.
All of these are virtually unheard of with retort pouched MRE meals.
Firstly not only are the producers watched like hawks by the government, but due to pouch being flexible and sealed they are often "boiled in bag" after production. This makes the MRE meals completely bio-sterile since there is no chance of post-boiling compromise
Second, as the foil layer is just a thin oxygen barrier and not structural, the internal seal is beefier and backed by a durable outter seal that provides structure and keeps the thin foil from being deformed to the point it would compromise. Thus there is nothing to rust, the inner seal is designed for high temperatures.
So the only real enemy of an MRE is oxygen - and the food inside being sterile-but-rancid.
So lets talk about Oxygen a bit.
We cannot make a true gas-impermeable barrier, something will always get through. Without getting into atomic physics, solid matter is mostly empty space. If you think of the Sun as an Atomic nucleus, as the size of a proton basically, The Sun-Hydrogen Atom's single electron orbit would be well past the orbit of Pluto.
The best you can approximate is by using a ton of extremely dense material to keep random gas molecules from being able to get very far through a material, making is so unlikely they are able to get through that its functionally impenetrable.
This matters when we're talking thin membrane MREs and earth's 21% oxygen atmosphere.
Oxygen will get in. The double packing will help, the oxygen absorber will help, but rolling the dice over 20 years means bad things are certain to happen eventually. You would want somethign to actively displace the oxygen, and you'd want to surround the MRE in highly oxygen hungry materials. Some anti-moisture stuff would likely be a good idea as well.
