AC I highlighted the last sentence in your post because I think this is an all too often overlooked point. When the defense does not have to worry about a RB that can instantly gash them for a big run, their safeties aren’t even needed to support the run.
Why is this huge? Because it takes away the power of play action passes and generally makes the passing game much harder to execute. In essence, with no RBs to threaten a defense, all the pressure to move the ball falls completely on the QB.
This type of weak running game creates more of those dreaded 3rd and 8 type situations where even great QBs are only about 35% successful on 3rd and long. 3rd and 2 is so different than 3rd and 8.