This seems like a bold claim to throw out without substantiation. It is undeniable that older devices are more resource-limited, and do run the latest OS faster. Sure, Apple probably is trying to push sales of new devices (which is their right—they have no obligation to retrofit the latest-and-greatest features to old devices); but to say that's the only reason seems rather strong.

The Air 2 has twice as much ram as the Air, and memory has long been the constraining factor on the iPad.