I think you may be confusing it for something else, magnesium chloride is literally a salt so there's nothing to degrade, and it's not going to react with air (if it did, it'd release chlorine gas).

Like sodium chloride, the most it will do is grab moisture out of the air and try to recrystallize into bigger clumps.

Edit It occurs to me you may have mean "It doesn’t lose potency over time!", in which case, true!