You have to remember (or if you didn't live throught it, consider) the circumstances.
The Patriots were a distant fourth in the New England pro sports scene. From 1966 (when the team barely missed out in playing for the first super bowl) through 1975, with a couple of minor exceptions they were unwatchable. Plunkett's first season brought excitement, as did a 5-0 start in '74 - but the team hadn't been in the playoffs in over ten years.
The 1976 season created a huge buzz in the area. The amount of change in attitude about the team was incredible. Yes, those Fairbanks-era teams never won a super bowl. But the increase in amount of positive attention the team was generating was amazing. You can't just write the '76 team off because they didn't win it all. The collective attitude about the Pats was a remarkable change.
Yes, the Patriots of 2001 and beyond did win it all, and had an historic run of excellence. But those Patriots had just been to the super bowl five years earlier. The fan base and interest was already there. The Patriots of 1976 did not have that foundation to build upon. Despite the Sullivans, they rose from nothing, defying odds to win fans of the Sox, Celtics and Bruins who were already used to winning with great stars, playing in the city in famous stadiums/arenas.
It cannot be overstated what a huge leap it was for the Fairbanks-era Patriots. I consider myself very fortunate to have lived through it.