Selfishly, because I don't want to sink money in to a 3rd AW! If they come out with amazing new features then I'm the type of person who will want to buy it. If the update is minimal then it's easier to feel better about keeping the one I have. That's kinda how I approach all Apple products, and the same reason why I haven't bought an iPad Pro since the Air 2 works for me.
Well, absolutely. But then it's really just your brain being prudent -- not selfish. Your heart says bring on the tech. It's up to Apple to which of those wins the battle over our wallet.