You have to consider a whole range of things. How corrosive is the propellant? Were the primers sealed/coated? (Humidity can degrade coatings.)
Have the primers bulged? Is is a Berdan or Boxer primer?
Storing ammo with leather and copper can cause the copper to corrode. Acid or veg tanning in the leather can give that green patina very very quickly...I have several examples of this. Storing ammo in a leather cartridge belt or pouch isn’t a good idea, no matter what the tanning process.
Lastly, I don’t store dynamite so I don’t know...I only know about the rounds I have from 1942 and some from before that and what I’ve seen over 30 years of storage. The best thing I did was cool dry basement and the humidifiers...
We agree on nearly everything. I have seen corrosion caused by leather. Often leather has lots of salts in the tanning process.
I think it is worth diferentiating between the degradation of ammunition and the potential for detonation.
It is my educated opinion that modern propellants will only detonate if they are improperly stored in a strong pressure vessel.