As someone who has spent a lot of time defining and enforcing processes (yay), it is far easier and more understandable to adopt a hard rule for this sort of thing. You don't want people to be debating whether or not their backpack is the acceptable size to include, while someone else's laptop case is not. Then you have someone saying that they are allowed to take their backpack, but it is in the overhead bin one row behind them, so could you just pass it to me...

For life or death situations, you need to have short clear rules that are easy to follow.

Remember, you will only actually lose your stuff if the plane is destroyed. If the plane is in the middle of being destroyed, just get out and be glad you are still alive!