A lot of teams think they have depth and interchangeable parts, which is what coach Dolan thought after Heps, but that assumption is irrelevant at nationals. Swapping out a 30:30 Terre Haute guy for a 31 minute performance can cost a team 50-60 points, whereas at Heps or regionals that might not make a critical difference.
Assuming they have a couple of elite runners, a team that wants to run well at nationals needs to get its journeymen scorers in at 30:40 or better, because after that the points added to the team score spikes and over the next 30 seconds it becomes a total clusterf$ck. These runners are all pretty much in the same tier, but the question is who is having their best race of the season.
Princeton's season could be as "Stanfordesque", where there was enough depth to cover some illness and injury problems mid-season. But those problems left a team that was going to be an also-ran at the NCAA level, because the guys covering are going to be on the wrong side of that 30:40 tipping point (or thereabouts), even if they are OK regional and conference runners.
My guess is they knew they were done as a top ten team right after Heps, considering how close it was with Columbia.