To give you an idea of how good the Giants were from 1964-80. They never made the playoffs and had I believe had two winning seasons. The worst team in the NFC East in the 1970s. Again one winning season and year after year competing with their arch-rival the Philadelphia Eagles for last place in the NFC East. Two of the biggest markets and cities in the country and two of the most storied franchises in the NFL and yet they were consistently competing for last place in the NFC East. I think the problem with the Giants of this era was that they fired Allie Sherman too soon after the 68 season and then not finding a good head coach for them until Ray Perkins in 1979. They had several different head coaches during this period that all had one thing in common. Losing season after losing season.
As great as Wellington Mara was for the New York Giants franchise he made a lot of mistakes in the 1960s and 70s. Not having the right general manager and head coach in the 1970s and poor drafting set this franchise way back. Also not finding a replacement for Yankee Stadium which was really a baseball park that the Giants shared with the guess who. All of these things that contributed to the Giants essentially being asleep as a franchise especially in the 1970s. Even the Chicago Bears who were pretty bad in this period as well-managed a couple of winning seasons and made the playoffs in the 1970s. But they did make a few good draft picks in the mid and late 1970s like Harry Carson, George Martin and Phil Simms that set them up well for the 1980s. But by in large the 1970s was a bad decade for the New York Giants.