The 5 inning rule was another one that made sense years ago. If a starter went less than 5 innings but the team held the lead the rest of the way the starter was usually out due to injury, or maybe ejection, or maybe the score was 12-8. The reliever that came in would have likely finished the game and would have been more deserving of the win. You wouldn't have a 4 inning start followed by 5 relievers pitching one inning each.