The compression does not wear your springs wear out. There could be some creep over time, but alternating stresses are actually what destroys a metal.
My second tour in Iraq we did an experiment loaded 27 rounds into a 30 round mag and left it for 7 months. What we found out was it did indeed fail the spring was compressed about 1/4in compared to a mag that hadn't been loaded. Firing with the mag we had several double feeds. Eventually in time they fail.