I can sort of see the argument that people won't want to hang around for ages. However, something I've noticed at many international parks is that, unlike UK ones, at opening time in the morning the crowds tend to be considerably less. Guests instead arrive in a flow throughout the day.
My guess would be people think "Oh, the park's open until 9? We can turn up a few hours later.". Especially with families I'd say this offers a good plan for their day. People would probably want to avoid an early get up in order to arrive at 9, especially those traveling long distances.
But think as well from the parks point of view. A smaller morning rush allows the staff and facilities to build up steadily, rather than everything suddenly being put under massive pressure, and issues arising as a result. Towers Street plaza, while offering the capacity, is naff at handling the big crowds in the morning if you ask me. People are everywhere, and it's a bit of a nightmare to try and get though sometimes. If they could spread the amount of people arriving more steadily it would really help. The same goes for the Monorail, at both opening and closing times.
I can see both sides of the late opening argument really, but I do think that if a park was willing to take the leap and stay open frequently as a matter of habit others would follow suit, and visitors would be more likely to stay for these extended periods. I really do think that guests would stay later. You only have to look at the lawns on a nice day at closing time to see people sat around on the grass enjoying a few extra minutes in the park even without the ride's operating.