The people who know about this- people who design and actually build stats will tell you the ML's mylar diaphgrams lose conductivity over time- the process of making the mylar conductive as used by ML simply wasn't as good as other stat manufacturers.
Hopefully they are now using a process that lasts a lot longer.
It is possible to apply a better material to aged panels and fix them as good as new, but seems to me that a lot of money has been made by replacing the whole panel. Why the ML community has taken this without a whimper is mystery to me. Seems it's easier to blame the environment than poor manufacturing.
I have 3 sets of ML's , love the sound, but dont like ML's attitude- they should have addressed this problem long ago.