That is faulty logic. The greatness of one area of offense does not diminish the other. The Greatest Show on Turf and the Cowboys dynasty both had the ability to run and pass.
I think this is the hardest point for a lot of fans. If your a team that focuses on a physical style of offense featuring the running game, don't expect a lot of success in passing. Maybe counter intuitive, but I think it has proven itself over and over again. Same thing in reverse, if you focus on having a top passing offense, you will probably struggle in the running game.