I'm not doubting that Lombardi's Sweep was a dominant play at the time. But like most things that are dominant someone sat down, did the math, and figured out a way to defeat it. If they didn't, then every team in the League would still be running that play. The guy that came up with the "solution" to the Packer Sweep was Tom Landry with his "Flex" Defense.

 

In Landry's Defense, instead of the Defenders crashing forward and chasing Running Backs from behind, they would mirror the pulling blockers and meet them at the play squeezing down the running lane and allowing the Middle Linebacker free to roam to the play. You can read a bit about it on Landry's Wikipedia Page under the "Great Innovator" section.

 

The counter attack to Landry's Defense was the Zone Block where it didn't matter if the Defenders were slanting, stunting, or pulling. The blockers simply identified their assignment and if the defender looped away from them, they moved to the next target.

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