Gambling, and fantasy football have done alot to keep the fans engaged over the years, even as the on-field product has suffered from rule change after rule change.
That, and the fact that the NFL never truly has an "off-season" anymore. They now have the preseason, regular season, post season and non-playing season. From the 0:00 mark on the scoreboard at the Super Bowl until the opening of training camp, the league, with the help of NFL Network and ESPN, has been able to keep football in our face with some kind of event or deadline every month of the year.
As kids growing up, how many of us over the age of 30 ever even knew that there was such a thing as the combine, and various pro days at the different draft prospects' schools? We knew about the draft, yes, but only as an obscure listing that popped up one Sunday morning in the sports section of the news paper every spring. Most of us were too busy paying attention to the beginning of the MLB season or following our NBA teams down the stretch to the play-offs to really care. Now it's all football, all the time.