I am not saying we shouldn't ant better, or possibly need better.
My feeling is when you go with the "smaller' lb group, you need more beef in front of them. I think the mistake that was made early on is we seemed to go with the lighter, quicker DT.
When blockers were 240+ lbs, they could take on guards...who were a bit lighter then. Now that pretty much all guards are near 300 or more lbs, going with lighter backers doesn't work. There is a difference between shedding a block to get around a player v running away from them.
Just the evolution of football. Just as the O's have transitioned in to more of a speed, passing and QB type running attack, the D's have had to make changes as well.
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