Funny, I found a fully loaded SIG P226 mag that had been in storage for more than 10 years (likely closer to 15) in a sealed bag and the spring was markedly weaker than the others that I had. It could be a one-off, but I am skeptical.
That's probably not a concern but your skepticism is well warranted.
I do enough NDT on springs in mining and conveyors and the failure modes are basically the same and here's how it goes.
Starting with a properly engineered and built spring ( proper alloy, wire size, dimension and geometry with heat treat and plating if desired)……
The spring will eventually fade and fail due to cyclic fatigue- being constantly loaded as in a magazine is only half a cycle so really doesn't degrade the spring EXCEPT
if the load is above the design limit. Sitting under a load ( within design) wont hurt it because it allows for that range)
Set (or 3 coil clashing)- this comes from rapid fire or excessive cyclic loading of a spring. Very common on car suspensions and rock crawlers, rock crushers in mines, vibrating conveyors in lumber mills and so on but not common in a weapon magazine
Beyond that, most spring failures result from other factors such as plating peeling, tooling marks, corrosion (mass loss)
Without actually measuring an unstressed spring then loading it-you really cant tell anything. This is why a lot of springs get replaced needlessly.
Try a good cleaning and proper lubrication first. ( and check the spring for wear/damage)