You have the park to yourself? Your tickets cost the same every year? You don't eat any food? because every buffet you eat in June and the start of January costs $4 extra per adult than it does the second week of January. It also makes sense Disney would close water rides in the coldest month of the year- when the demand for getting drenched is a bit reduced.
A second thing I'm saying is that this thread is sort of silly. Crowds are just one factor WDW considers when scheduling rehabs.
1.WDW doesn't have enough expert staff (engineers, artists), equipment, planning, work space, etc. to rehab all their rides at the same time.
2. Not all rehabs are the same. If WDW needs to replace a mass produced ride like Dumbo, they just order it from the manufacturer, and swap it in a matter of work hours. Custom castle repair is a bit different.
4. Weather is a factor. September is slow in large part because it is also high hurricane season. Seems pretty obvious to me that hurricanes limit rehab options.