I don't know about the "destroying" part, but you can bet the goobermint won't sell them on the surplus market. They will probably be stored in big fenced-in piles for 20-30 years first. Same thing as the "Korean" Garands the congress ordered brought back to the US for sale to civilians. Seen any action on that yet? :(
DoD tried about a year ago for a voluntary turn-in of Pmags by the troops, which was mocked. Now they say slight dimensional problems with a new experimental USMC machine gun means all Pmags must be surrendered. Never mind that Magpul has already been shipping the Pmag V3 to fix that very problem.
Magpul wants to keep millions of troops and civilians happy. DoD just needs to keep a couple of big contractors of aluminum magazines happy.
New aluminum mags seem okay, but after a few years of heavy use, are no longer seen as trustworthy. Pmags seem to go on forever, and if one goes bad, it's pretty obvious, because it's broken into pieces. No guessing about the feed lips remaining in the proper shape, or if there are subtle dents in the aluminum body.
I know there’s a gap between what the government shouldn’t do and reality but if those mags are personal property they shouldn’t be confiscating them. I get tired of repeating the phrase but ... that’s unAmerican.