No team that I can recall recently has ever won the Superbowl by building in Free Agency. The Skins keep trading away their picks and picking up guys that are always on the other side of their rookie contracts. The problem is two-fold.
First... you end up with very little depth behind your starters. Superbowl teams can take one or two major injuries and still perform well. Look at Pitt last year at Running Back. The Skins are very thin across both lines, LB, CB, and Safety. To get under the cap this year they cut several vets in those positions.
Second... It generally takes guys one or two years to really start to excel in a new system. Picking up unrestricted free agents means you are getting guys that already have between around 5-6 years in the league. Adding two more and you are dealing with guys that will only have 1 or 2 years of peak performance for your team. After that they are sliding down the wrong side of age 30.
I'm not saying that the skin's haven't improved this year, but they continue to do so at the expense of developing new tallent. That is their "cap hell". To always be one injury away from a 45 year old Starter. I don't think there is anyway the Skins could have dealt with the injuries the Cowboys faced along the Offensive line for instance (Rogers and Flozell). Cornerback might have been their deepest position last year (after QB) and even there they were exposed. The same is true for SS (where they were the worst in the league), DT, DE, and WR.