1) You bring up what would be an excellent point if BYU had average athletes in the defensive backfield. But...
2) BYU has very good athletes in the backfield and often they have excellent athletic skills.

EDUCATION SIDE BAR - Every player's speed is a product of far more than his athletic metrics (height, weight, explosive thrust, foot-speed, and in the case of defenders, stride speed). Those physical factors can determine POTENTIAL, but not player speed. Player speed is very heavily dependent upon decision making or mental speed. Muscle memory is every bit as important as athleticism. The mental game is earned in the film-room and with reps against the scout team players that represent the individual opponent's favorite moves, down and distance tendencies, etc. So real speed has as much or more to do with game prep as athleticism. BYU is NOT slow in the backfield. But like any group of players they can make a mental mistake and can get burned.

In order to blitz effectively, the DB's need to study film of the guys that are likely to block their path to the QB and the QB's moves and tendencies when avoiding a sack, and his mistakes when he fails at it. Again, the film room is where game speed is developed. you can have a slew of 4.4 40 guys (and BYU does) but if they do not practice X-ing the LOS much, they will not be good at blitzing or stunting to the QB. The same is true of Linebackers. Having said that...

3) BYU does NOT use pressure enough on 3rd and long situations. Rushing 3 and dropping eight is easy to pick apart because after 3-4 seconds, someone has shaken the coverage underneath and will be open. Or any QB with decent feet will know where he can run. It's not that you never want to do this, but BYU does it way too often. Now the question is how do you blitz for sacks. Well UTAH wrote the book on this under K-Whitt and BYU should be following his lead because nothing is more demoralizing to an opponent than when the QB is sacked on a 3rd and long situation. But to get to the QB you need to do it with gusto. I hat the 5 man blitz because you weakened your secondary, the lineman will probably delay the attack long enough for the QB to get the ball out of his hands and its like a political moderate - you didn't really take a stand on anything. K-Whitt has been so successful over the years doing all out 7-go attacks on especially 1st downs and on 3rd and over 5 yards. Remember, Whittingham was one of the best LB's in BYU history. The dude knows when teams were successful against some of the greatest QB's to ever play the game because he was there.

4) So yes, BYU should blitz more, should be taking more risks and should be especially proactive on 3rd and long, as well as 1st downs inside the 30 on both ends of the field.

SECOND EDUCATIONAL SIDEBAR - oot (OVER-TH-TOP) completions are actually pretty rare in football. With an increase in blitzes comes an occasional increased risk in giving up a TD. If you track K-Whitt's teams, during the Bronco Mendenhall era K-Whit's defenses only twice out played BYU's in major statistical categories. The deltas were not huge but apparent. Yards surrendered, points surrendered, everything across the board tended to favor BYU even when BYU was playing 2-3 P5's a year and UTAH was in the MWC. But, UTAH frequently lead the nation is sacks and forced turnovers. Turnovers always turned into more defensive points per game and more short field drives. UTAH turned risk into reward and they still do. I have always said, you cannot win a battle in retreat and you cannot win a war being passive. BYU NEEDS to play much more like UTAH. BYU has the athletes. I does the film room well. It just needs to be more aggressive on 2st and 3rd downs and then occasionally on 2nd and long to keep it interesting, and especially when the opponent is facing 85-65 yards of pasture before the goal-line.