As someone who lives away from the NE, meeting people, inevitably talking football, and saying you support Sunderland, is a sense of identity. Sure I take a bit of stick, but more times than not, it leads to discussion. Supporting Sunderland now, is saying you are a genuine fan, with a pride in the area you come from.
Once you get plastic fans, and kids all over the country wanting the shirts, saying you support Sunderland would have less meaning. You'd just be another gloryhunter who has to justify why they support the team. In the same way that when you meet a Man United fan in a pub in the NE, your reaction is not "I bet hes from near Manchester and goes to a few games a season"