Mr. Premise
The Living Force
Angelburst, tobacco if stored properly should improve for years. Pipe tobacco twenty or more years old in unopened cans fetch high prices. What I think may have happened with your American Spirit is that because it's additive free they don't add propylene glycol to it. What PG does in keep it moist and inhibit mold. If you don't use PG then water is added to keep it nicely moist. But you don't want to store it that moist for long periods. So in the future it would be a good idea to open the cans, let the tobacco dry out, then store it in canning jars.