+1 to Springs wearing out from work... not compression.
Do an experiment: Buy a new mag for your gun, load it, label it and stick it in the back of your safe for 2 years.
Then, label another one and use it and only it exclusively at the range for all practice.
For safety's sake, take a 3rd mag and stick your defensive ammo in it, and use it as you feel it "should" be used. Carry this magazine if you carry.
With the range mag, unload/reload it a couple times a day.
Do this for a couple years, then take the range mag and the safe mag to the range and see which fails first.
I used to fiddle with my Glock 21 mags, and I wore out the springs in my 1 pre-ban magazine (I got it in 2001) from loading/unloading over a period of about 3 years.
My 10-rounders that just stayed loaded and saw light range duty never wore out. I noticed that spring fatigue was caused by cycles and not compression.
Since then, I've got 4 mags for my XD9: The 16 round ones stay loaded with defensive ammunition and get tested very rarely. The 15 round ones (it came with 15's) also stay loaded, but with range ammo and get emptied repeatedly when I go to the range.
The 15's are a bit looser than the 16's at this point, but both work fine. I could probably wear out the 15's by sitting at home and loading/reloading them.