It hasn't always been great. But for the last 30+ years, which is not a short period of time, it has been superb. Several teams haven't even existed that long. That's a lifetime for some fans and many people on the board. There is no team, none, that is unaffected by fielding truly awful teams (including the UConn Huskies). Not the Red Sox, not the Yankees, Redskins nobody. Even the Cubs...who know losing better than most.
Saying that the Patriots have fair-weather fans based on 1989 is absurd. That stadium was so freaking miserable that having anyone show up in mid-winter to watch a bad team is pretty good. They don't have fair weather fans now.
Using the magic of Google, we see that the Giants (predictably) have the largest waiting list for season tickets, followed (predictably) by the Packers. Next are the Saints, Patriots and Denver (over 60,000). Half the teams have no waiting list. Many have a very small one.