This is good news in a way, as it will affect their ability to recruit significantly.
Also, Hitler had a 'no retreat' policy and it was a major factor in his defeat. If you cannot retreat, you have just lost 50% of your ability to maneuver and find strategic advantage. Imagine trying to play chess if you can only move forward. That would be a rough game.
I understand the macho 'fight to the death' point ISIS is trying to make, but it is amateurish nonsense. It will hurt them more than help them. You can't make a blanket rule like that for every situation. It doesn't work that way.