Detroit's population has been steadily declining since 1990.
I don't think there's a correlation. Though they made the play-offs the team was quite mediocre, witnessed by Barry Sanders' abrupt retirement
"Several years after retirement, and repeated refusals to discuss the abruptness of it, Sanders finally admitted that the culture of losing in the Lions' organization was too much to deal with even though he said that he could still play. He explained that it robbed him of his competitive spirit, and he saw no reason to believe things were going to improve. Although Detroit had made the playoffs the season prior to his retirement (losing to the Tampa Bay Buccaneers 20-10 on the road in a 1998 NFC Wild Card game), Detroit drafted quarterback Charlie Batch in the second round of the 1998 NFL Draft. It became apparent that Batch would become Detroit's full time starter the next season, and Sanders seemed unwilling to embrace yet another change in the Lions' seemingly endless carousel of quarterbacks and offensive philosophies. He had also gone on record to criticize Detroit's front office (most notably Chuck Schmidt) for releasing Pro Bowl center Kevin Glover for salary cap reasons."
I'm pretty sure if a correlation between Lombardi trophies and economic decline existed, Detroit would have won at least 5 or 6 of them by now.