We will not know if MREs expire for at least another 50 years.
Up through 1943 the Army was still fielding Iron Rations from storage after The Big One. There are numerous reports that the Japanese especially liked them.
K-Rats were finally falling apart (literally as well as nutritionally) just before Vietnam started. Most likely due to ove-handling by successive generations of soldiers and civillian warehouse personnel looking for Hershey's Tropical Chocolate bars that had been missed by earlier waves of scavengers. The last bar I ate was back in the 1990s - it was still not merely OK but goooood (for all values of Hershey's Tropical Chocolate).
C-Rats are still in storage. The only reason nobody seems to be scavenging the is an overpowering fear of ending up with a can of Ham & Lima Beans. (Historical side note: Dad's primary job in the Quartermaster Corps/Research and Development Command was to figure out how to keep the fat mixed with the other stuff in the can. Seems Nature can be a real biotch, 'cause regardless of what they did/put in there a hockey puck of fat would appear at the top after about a year. The solution was to invert the cases every 6 months. Took one of the woprld's top three researchers in lipoprotien stability to come up with that!)
What you need to watch out for are the surplus squad meals
http://www.longlifefood.com/MRE-Tray-Packs/products/1029/ They are NOT just more MRE stuff, in spite of what they retailers will try to tell you. And they do NOT keep "just as long" as MREs. 24 months and you start to see both nutritional and physical breakdown - yes, even of the pork sausage patties, hard as that may be to believe.
stay safe.
