Does keeping a magazine loaded weaken the magazine spring?
More stress is probably placed on a magazine spring by shooting only one full magazine at the range than in keeping the magazine completely loaded for ten or twenty years.
The shock absorbers on a car or truck are not likely to suffer much damage if the vehicle is kept stationary ~ regardless of the load to which the shocks are subjected. Putting the car on a lift when it's parked won't do much to prolong the life of the shock absorbers.
If, however, the vehicle is taken for drives on very bumpy roads, the shocks become stressed and may in time suffer enough damage to warrant their replacement.
Magazine springs, similarly, are insignificantly stressed by keeping the magazine loaded. They are much more stressed when the pistol is fired. Unloading the magazine, rotating magazines, etc., is akin to parking a vehicle on a lift to spare the shock absorbers.
Magazine springs are one length when they are first wound. They take on a set and their extended length becomes shorter when they are compressed manually to their solid height. This second extended height is the same whether the spring is very briefly compressed to solid height or held completely compressed for a year.
Shooting a single magazine through the pistol will shorten the magazine spring beyond the above mentioned second extended height, whereas keeping the spring compressed for a year will not.
Shooting hot or heavy recoil producing ammo puts more stress on a magazine spring than shooting lighter loads. It is the bumpier road.