What renders chemicals unusable in general? Chemical reactions and physical deterioration. The latter one is all about handling and storing. Although Spheradorb can safely be frozen (I don't know about Sofnolime) a repeated cycle will cause erosion and cause dust. Driving around with it in your pickup during the week because you are too lazy to unload it when you only dive in weekends is another bad thing from a storage point of view.
The chemical aspect is harder to answer. Some chemical substances are instable in itself and will eiher react over time or e.g. rearrange molecules to a configuration unsuitable for your purpose. (The different salts may separate over time especially when conditions are favorable which potentially makes it "start" the sorb during pre-breathing)
The good thing is that NaOH and CaOH are very stable by themselves. The packaging will have to be CO2 and oher accids proof. High Density polyethylene (HDPE) will do perfect. I can see no reason why it shouldn't last 20 years other than the canister itself aging over time and becoming permeable.
Preventing that should be our top priority. Try to store cool and away from UV radiation that ages the canister (no exposure to sun light) and the sorb should last "forever".
I'd love to know what I've overlooked here
BTW My sorb expires in 5 years. I'm interested in what people in different countries find on their canisters. I really wonder whether it is a "technical" limitation or just another way to limit liability. Especially different expiry times for the same product sold in different countries would indicate that way.