Unless you have some new physics or metallurgical research of which I am unaware, your “sense” of your compressed springs weakening is Poppycock brother. Given the properties of metallic atomic bonds it is neither possible nor probable. How did you measure the weakening btw?"Magazine springs are not weakened by compression" is a bit of an absolutist statement if you ask me.
While I do acknowledge that compression and decompression cycles weaken springs much more substantially than just leaving them compressed, I have found in my experience that leaving certain magazine springs compressed does in fact weaken them to some degree. My HK USP magazines being the most effected by being left fully loaded of all my other pistols. My theory is that the metallurgy or heat treatment of the USP springs I had were sub-par. I have many other magazine for other guns that I have left loaded for just as long if not longer and they dont seem to be weakened anywhere near as much as my USP magazines were.
I watched a video by Paul Harrell where either he, or a friend, bought a Browning A5 or similar type shotgun that had been left in a closet for a multitude of years with the bolt locked open. When they went to test fire the gun it had consistent failure-to-feed problems due to the recoil spring being weakened from being compressed for that length of time. The issue was remedied by replacing the recoil spring if I remember correctly.
I wouldn't say that it is universal but I am of the opinion that its certainly possible/plausible that leaving some springs (depending on their quality) compressed can weaken them albeit not as much as a high amount of compression/decompression cycles will.
I am a big Paul Harrell fan but he is dead wrong on this score.