I don't believe the AMA has declined because of strategic decisions they've made. Interests ebb and flow.
We'd probably all agree that the AMA's golden age was the 1950s to around 1990. Who were the big heroes of WWII? Mostly the pilots. Plus the space race was the cutting edge of technology and uber cool. Kids dreamed of flying, so they built models. Many of them grew up and kept pursuing modeling as adults, creating the healthy adult membership the AMA enjoyed until around the year 2000.
Here the news flash: The AMA was already in decline in the late 80s. Its median age was steadily increasing, and most of its youth growth was the children of current members. The AMA hadn't changed it strategy, but kids were scratching the itch to build and create in different ways.
I can say for certain, as a middle school tech and engineering teacher, there are still plenty of kids who love to build and see their creations work. But in the 90's the digital age had hit and so a lot of those kids turned to building website and learning to code. It's really the same type of person, just a different way to express those interests.
If the AMA is smart, they will start marketing to a fresh new group of kids that are coming up whose parents are reacting against electronics. There is a strong wave of unplugging and getting kids' hands busy, and parents are willing to spend money on it. I could see free flight and CL making a comeback, along with FPV and new autonomous creations taking off too. I'd say the AMA has the best opportunity now to create genuine youth growth since probably the 1960s.