What you say is true: when the O and the ST do well, they put the D in a better position to do well. The O can do well by holding the ball and forcing the D to have to face the team fewer times. Likewise, the ST can pin the O deep and give the D more opportunities to stop the other O before they score.
Looking at scoring O, we were 26th in the country last year. But doing that well was largely since we played two FCS teams - scoring 66 and 70. If you take either game and give us our average scoring of about 32 points for playing a real team, we drop to 42nd in scoring O. That seems about right.
I realize I'm playing with numbers; the number I like the best is Power Ranking for the team overall since all the parts fit together as you pointed out. Plus Power Ranking takes into account Strength of Schedule and Home/Away. By that measure, our team was 40th last year. http://www.jhowell.net/cf/cf2013.htm
The scoring D was 29th and much improved from previous years (65th scoring D in 2012). Playing the same game by changing one of our FCS games to a real one, our D goes to about 47th in scoring D. So the D is clearly getting better.
But back to my point / frustration, our O is not getting better and is usually impotent against top 40 teams.