I will agree with your last sentence and most of what you have said. But not all springs are created equal and some fail when under constant compression or extension. I highly doubt that Glock got a bad shipment of steel and made defective springs.FYI, the springs in a magazine do not wear out or lose tension when left loaded. Springs do not lose pressure when left in a compressed position or a extended position. All springs lose their tension as they work, in other words all the constant loading and unloading has done is to weaken the springs in your mag's not help them. Think of the valve springs in a car engine, how many are fully compressed when not running, how many vintage cars sit for years in storage then get a carb rebuilt fuel system flushed, oil changed and new plugs thrown in and driven. Mag springs are no different.
It is myth that leaving mags loaded hurts them.
Clean the mags, stop rotating them out and only unload them when you shoot them.
BTW, your valve spring analogy is good, but using a car's suspension spring works just as good. Quality springs last longer than a worked spring in most cases. So I will repeat "stop rotating them out and only unload them when you shoot them".
After disassembling and reassembling let us know how they function.