You can't have it both ways. If he's born in 1971, I doubt he became a Cowboys fan when they won Super Bowl XII in 1977 (early 1978). He was 6. But if you say his sports devotions began that early, then he rooted for a number of AWFUL Mets teams, including the one that season that traded away Tom Seaver.
So let's say his fandom started when he was 10 or 11. Then he rooted for some bad Mets teams that would soon start showing some promise, as they started to come together in late 83, then became a competitive team in 1984 (losing to the Cubs), and 1985 (losing to the Cardinals), before finally winning in 1986. Then there followed several years of them frustratingly underperforming.
Which would be pretty much the same of the Cowboys he started rooting for if he did so in the early 80s. Because those Cowboys went to three straight NFC Championship games and lost all three (to the Eagles in 1980/81; to the Niners (THE CATCH!) in 1981/82; and to then Redskins in 1982/83). Then they went into a decline that eventually led to Tom Landry's dismissal before Jimmie Johnson was able to restore them to championship level in the early to mid-90s.
So I agree his fandom doesn't seem to be a case of front-running.
FYI, I'm just a few years older than Sal and grew up in North Jersey and thus in the NYC media market, and my block had a number of Cowboys, Vikings, Raiders, and Steelers fans. You know why? Because the Jets and Giants were terrible and didn't sell out, and so their home games were often blacked out on TV. You know whose games they showed instead whenever possible? The Cowboys, the Vikings, the Raiders, and the Steelers. Many of those years the Jets and Giants might have had seven TV games a piece while the Cowboys, Vikings, Raiders, and Steelers were on constantly.