Sunday nights sometimes get close to empty as midnight draws near--many of those locals who visit the park that day (and some non-locals) have to be back to work on Monday.
...
I've never been to DCA for a super-late closing (I think 10 PM was the latest). But of course the wrong side of Paradise Pier (and much of the park) is practically empty during a 10:15 World of Color. Last trip I hung out there a bit after the show, to wait for the crowds to vanish, then slowly made my way back to the front of the park. It was practically empty by the time I finally left.
Even Cars Land did get LESS crowded as the 10 PM close drew near, and I'll bet Grizzly River Run would've been nearly empty!

But the park was pretty much going until official park close.