there's actually data from last season that suggests it actually isn't that difficult (completing deep throws vs modern shell type defenses), teams just aren't doing it
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The average depth of throw against two-high coverages was 7.8 yards in 2020. It has dropped all the way to just 6.6 yards in 2024. Offenses threw the ball an average of 1.9 yards short of the first-down marker against two-high in 2020. This year, they are throwing an average of 3.2 yards short of the sticks against the same coverages. In 2020, 11.6% of passes against two-high traveled at least 20 yards in the air. This season, that number is all the way down to just 9% of passes.
These aren't the overall numbers. They are JUST against two-high. Teams are getting worse at attacking these coverages because they are surrendering to what the defense wants them to do. Completion percentages against two-high are slightly up. Drop rates on those throws are down. The rates of off-target throws are down. "Take what the defense gives you" is a common theory of offense, and teams are taking that to heart -- but it's not working in their favor.
And here's the other thing: teams that do throw deep against two-high safety looks are seeing more success than ever before. On throws of 15-plus air yards against two-high so far this season, NFL teams have a 50% success rate. Quarterbacks have a 19.9% off-target throw rate, 8.1% touchdown rate, 5.1% interception rate and 35.3% explosive-play rate, while receivers have dropped just 2.9% of their throws. Teams are gaining an average of 13.3 yards per attempt and 0.65 EPA per dropback. All of those numbers are the best of any season since at least 2019...