I agree that in a design that doesn't heavily compress the springs, a good quality spring should not weaken when the magazine is left loaded. If, on the other hand, the spring is left at (or very near) 100% compression when the mag is loaded, it's very likely to weaken even if it's virtually perfect in every respect.
In all fairness, the fact that the magazines still function isn't evidence that the springs haven't weakened, it is only evidence that they haven't weakened enough to cause malfunctions. As noted, even with about a 27% reduction in spring strength, the Ruger P89 magazine will still lock the slide back when the slide is racked manually and I'm fairly optimistic that it will function in a range test even at only about 73% original strength. We'll have to wait until the test is completely over though to find out for sure.
It would surprise me tremendously if the G20 mag malfunctions in the final range test. It has certainly weakened (about 7% strength lost) but I doubt that's anywhere near enough to cause problems.