I guess Graco's been using the same plastics for over a decade now and has some historical data to work with (I bought THE first Turbo in Hawaii in 2002, man, what a thrill, hahaha). Those plastics aren't breaking down as fast as they predicted them to, and they keep making replacement parts.
Convertibles are lasting longer in general, climbing to 8/10/12 years, I expect boosters will start having longer lifespans, too, just not quite yet.