It is difficult to categorically say that the decline is due to a reduction in competitive balance. GAA attendance figures can be volatile due to the nature of the competition structure. The single-game knockout structure of the gaelic football championship makes comparisons difficult. A year where Dublin are eliminated after the first game means almost 30% of the population of Ireland don't get to support the county in which they are resident for the remainder of the Leinster championship. This is bound to have an impact on attendances. In a year like 2007, where Dublin are involved in replays, the numbers are boosted. One might think it is easy to exclude replays but that would only be so where the games are played as stand alone games. Double-headers are popular in most GAA competitions. Making comparisons between years is not easy.
The top graph shows the numbers that attended the 10 regular games of the Leinster senior football championship for the years 2002 to 2016. The complications that arises with replays and double-headers are adjusted for to the extent that it is possible. The lower graph shows the attendances at finals. It is a "cleaner" measure. It is a decade since 80,000+ went through the gates of Croke Park to watch a Leinster football final. Surely Dublin's dominance is contributing.