With most college teams passing the ball at least 50% of the why don't teams simply use 3 or 4 down linemen and fill the remaining positions with prototypical defensive back type players ( 5"10" to 6"1" 180 to 200 pounds) who are faster and in more abundant supply than your prototypical linebacker (6'1" to 6'3" 215 to 240 pounds)? I think it would make sense to sacrifice the size for speed.