…since the iPhone 6S screen cracks easily?
I don't know where you got this idea, but I have to reject it. I've had a 6s+ since September 2015 and it has never had a case or a screen protector. It's been dropped a couple of times, ridden around in pockets and cup holders and generally used as the phone it is.
The screen is not cracked, it is not scratched or in any way compromised. The phone itself could pass for brand new. I take care of my stuff.
I do not agree that the screen cracks easily. If that were the case this would have happened by now and it hasn't.