Any mechanical or structural engineer will tell you that springs DO take a "set".
There is no debate about it, its an easily tested and observed physical phenomenon.
It has been incorporated into engineering design for probably 200+ years.
You know how a piano or a guitar loses its tune and the strings need to be tightened? That's due to strain relaxation, aka "spring set".
The amount of spring relaxation or creep depends on certain variables like duration, temperature, compression, etc.
If you want to read about the general principles, see the following:
https://en.wikipedia.org/wiki/Stress_relaxation
https://en.wikipedia.org/wiki/Creep_(deformation)
The actual pertinent question is:
Were the magazine springs properly designed to allow for "spring set"?
Obviously, in most cases they were designed properly and the spring set does not affect proper function of the magazine.
Taking a wild guess, I would say that a mag spring left loaded for a real long time would lose around 10% to 15% of its compression force.
If the guy who designed the mag takes that into account, then it won't be a problem.
Please cut and paste the above into every thread where a bunch of guys insist that spring set is an unknown enigma of mysteriousness.
Eventually it will filter down to the lowest gun shop bubba, but it's going to take around 50 years.