What you are missing with this theory is why the lower seasons are lower. It isn't because it just never occurred to anyone that September or the week before Thanksgiving might be a good time to go to Disney World. The lower times are lower because of how people live their lives - when kids are in school, when many professionals are able to take vacation, when sports or activity schedules allow, when the weather is best, etc. More people are willing to go in the summer, when kids are out of school, than in September, when they just started. The Grand Canyon is much less crowded in November than in July. Flights to Europe are less expensive in January than in June.
So if the economy is good and the discounts are good, people will travel. But most of them will travel when it works for their families - and seeing "September is a low time for crowds" (which shouldn't be a surprise to anyone) probably isn't going to make someone pull their kids out of school if they weren't doing that already, or if the kids don't want to be pulled.