The rotation method seems like good practice and I agree. But valve springs are a different animal plus they are subjected to more extreme conditions than a magazine. Valve springs go through extreme thermal cycles (+-200ᵒ) that may affect their resiliency compared to (mostly) ambient temp magazines.It makes good sense to do so. I do not believe that any spring left compressed for too long will maintain its resiliency. I have seen this in race engines that have sat for a long time and valve springs lose a step if they are left compressed. This is seen if checked. Its why they need to be cycled.
Yes, cycling does step up the process but leaving compressed is also a detriment. How much is really hard to say but the springs in my 2011 race mags were junk if I left them loaded too long and that would have been a couple of months. Happened 2 times with several mags I did not dump when I left them stored. At the price of those springs that's ****ing nuts.
Your long term experience with loaded mags is noted/acknowledged though and I will be paying attention to how my mags are stored and their performance over time.
My Browning pistol/mags have been loaded for over 20yrs now and no noticeable effects so far.