Look, that doesn't make sense.

The Mini 4 is already behind the Air 2 in terms of hardware. If you are worked about the Air 2 being out of date, you shouldn't be thinking about the Mini 4, either.
Or in other words, no, I don't think that's a reason to not buy an Air 2 today.
I had an Air 1 until early summer. Honestly, if you already own it and don't care about split view, upgrading wouldn't make sense. As a new purchase today, on the other hand, I don't think the Air 1 makes much sense. It's on the wrong side of the 2GB divide.