Are we perhaps exaggerating the attendance decline too? When people talk about the "glory days" they're probably talking about days before I first got on campus in 2009. Being from Illinois, I had never heard of Plattsburgh or Trinity or Middlebury. But, as a freshman, I remember being really impressed, almost shocked, at how big of a deal DIII hockey was and how big the crowds were. I vividly remember during the 2009-2010 season standing two or three rows deep at the railing and watching the Cadets slaughter people.
So I looked up the attendance from some of those games I remember (based on score) and I guess it was in the last year of the interlock since they were almost all against NESCAC teams. Hamilton, Wesleyan, and Trinity drew crowds of 1400, 1400, and 1700, respectively. Plattsburgh, played at the Primelink and in early January when the students were gone, drew 1000 and 1200. Only big difference I see is Castleton and Middlebury being sellouts, where last year both drew below 1000.
But, given historical context, 1724 against a conference contender but not a traditional rival seems to be rather in line with past numbers.