Well, back in 2013 when the first Air launched with iOS 7, they probably did think 1GB was enough for it considering Apples typically conservative use of RAM due to iOS efficiency.
Skip forward a couple of years and two complete iOS revisions and it's only natural that what was once the bleeding edge as far as iOS devices go is now not so cutting edge.
No one builds devices that are going to have feature parity for the duration of their useful life, it's not a good business practice, nor is it cost effective from a manufacturing perspective. No hardware from any manufacturers get every new feature for ever, there has to be a cut off to keep the system running well and of course to encourage users to upgrade eventually.
But these older systems can still function really well. The Air 1, mini 2&3 and iPhone 6/6 Plus all have 1GB of RAM and still function well. We just can't expect them to get all of the new toys forever. I'm still using my 6 Plus, having decided for the first time ever to skip an iPhone and wait for the 7, and honestly, as much as I'd quite like 3D Touch I'm still not being drawn to upgrade (we'll see how long it lasts

) My 6 Plus has still managed to do everything I've thrown at it.