On TURN 5 ONWARDS? You might as well play Last Gamble at this point because one of you is probably dead by this point and even if the duel is not over the amount of cards in hand will probably be small at this point
Never mind reaching turn 5 to swap your hand, if you get a bad hand since the start you won't even survive past turn 3.
I think this is going to be their attempt to get rid of Restart. After all, being able to mulligan on your first turn is always going to be overpowered, since it lets you get to your setup should you not draw the right cards. By the fifth turn, however, most of your hand has already been played, so you're often just shuffling back one or two cards. That, and you're banking on IF your setup didn't get overrun by turn four.