The odds of them devaluing due to this promo alone are slim to none. I've seen people do the math in other comment sections - AA has hundreds of billions of miles outstanding, so even if there were millions of dollars of donations made here (which is itself unlikely, IMO), it's basically nothing in the big picture. (This source is somewhat out of date, but they note that AA had over 853 billion miles outstanding in 2015: https://www.traveldatadaily.com/american-ffp-analysis/)

That said, if you bought millions of miles through this promo, I'd be careful not to gift them to friends/family/etc. at least for a while. It wouldn't surprise me if AA is watching the accounts that bought that many (there can't be that many tbh) to make sure they aren't reselling them. I can already see the future blogs about AA shutting down the accounts of a few people who say they bought millions of miles, were shutdown, and claim they only transferred a few thousand to "friends"...lol.