Apple has announced that iOS 9 adoption has reached 75% as measured by the App Store on January 11th. That's up from the 70% adoption level reported on November 30th.
Of users who are not on iOS 9, 19% are on iOS 8 and 7% are on an earlier firmware version.
Every new apple product always runs a new released iOS like no other. Releasing iPad Air 3 like this will just make people think it's a minor update. When it comes to new products, there has to be a fresh new iOS version to support new features.