Around what time do you think the mainstream interest in boxing started to fall away?

Good point, back in McGuigan's day you had 3 channels to pick from

No wonder every bugger tuned in on a saturday night !!

 

Two factors:
1) When ITV stopped showing live boxing. When try did the big fights were accessible to all and therefore were watched by large audiences
2) As the better boxers get paid more and more money the interest in them dwindles. By this I mean, you need the best fighting the best on a regular basis. Now fighters do not fight frequently enough and they cherry pick their opponents thus robbing the public of frequently completitive fights.