Can one show that a majority of bridge hands could have been successfully defended?
What level of play are we talking - world class or novice or somewhere in between. 

Any player who is making or defeating an inappropriately large, or small, percentage of contracts weighted by point value is inefficient by definition, and thus is a poor player. 

Good players make and defeat the right number of contracts, weighted by point value (for the scoring type - match-points, rubber, IMPS or board-a-match), because that is the definition of competent play. Given the close competition of most world championship finals, those players are all efficient, and thus actually good players. 

Update:
It's widely accepted that defense in Bridge is more challenging than offense. Is that not semantically equivalent to stating that, double-dummy, most contracts could be defeated (by perfect defense). Only a naïve or extremely weak declarer is not going to bid aggressively, on the premise of taking proper advantage of the greater difficulty of defending.