I feel like the whole operating hours issue was never fully resolved, but this quote kinda has a lot to do with why I think Universal is playing it the way they are. They don't want people splitting up their days or going in segments (8 am - 4 pm, and 4 pm - 11 pm). All that time where people are either a) resting or b) leaving early/arriving late is time where guests aren't spending money in the parks. Giving them all that time in the park gives them plenty of time to eat outside the park, preoccupy themselves with something other than shopping, etc. 8 am is kind of early and 11 pm is kind of late...those times have the potential to be dead and that's just a waste of money.
The other thing is the lack of novelty the expansion has--the hype surrounding HP 1 was far greater to the general public, IMO, because it had never been done before. I think the urgency people had to see the first-ever Harry Potter theme park experience is gone with HP 1.0. Now, people will probably be more willing to wait for their regularly-scheduled vacation to roll around to see the expansion. So it's not like the parks should be *as* unbearable as they were in 2010 anyway. Of course, even if they are, the point still remains that shorter hours means less motivation for people to leave the park and more motivation for them to stick around, shop, etc.
I also think that with the addition of Cabana Bay, Universal is really pushing multi-day visits...if they allow you to get the whole resort done in one regular operating day, there won't be as much need to stay at the hotels.
Of course, this is all baseless speculation on my part, but I agree with JungleSkip on this issue--Universal has largely always been a park that empties out around 5/6, no matter what new attractions are there. Because at the end of the day, it's still much more "local" in feel than Disney (the percentage of visitors is less die-hard tourists and more of locals who have the mentality that they can come back anytime). Of course, if Universal turns out to be completely unprepared, they can always just extend the hours as the summer goes on. But as of now, I don't think it's a huge deal.