This is a bad idea because attackers might just chill knowing they won't lose anything to auto-retreat, there has to be an incentive to make attackers actually attacking.
I believe the troop attrition cost is still in effect. If attackers go too long without getting a kill, they lose a certain amount of tickets every minute. However, I can't verify this is actually still in effect because every battle I've seen to date, they attackers never wait that long. It will be more obvious when castle seiges pop up. That's where you really see the delay in attacking