The latest update to iOS 9 is, as far as we know, reasonably secure. It's hard to say what Apple will do if a major security exploit should be discovered for iOS 9 and/or older devices. They may offer another iOS update just for older devices (they've done it before for the iPad 1), or they may not. It will 'probably' depend on how sever Apple thinks the risk is (and how many older devices are still active).
But for now, you should be fine.