Canceling the shows saves them money, and allows them to help justify the early closing of parks, which saves them more money. If the early park closures were for allowing enough time to adequately clean off the attractions, they would not be open continuously throughout the day.
I don't see why the parks can't be open until like 9, at least. As mentioned, the 7pm closure would make it so everyone leaves at that time...just like party nights in the Fall and Holidays. Closer to normal hrs would help spread out traffic.