If parks reach capacity, they will do so early in the day and then they will begin to clear out by late afternoon. If you park-hop, just realize that you may have to hop to a park you weren't planning on hopping to! But even then, you can usually get back in if you just wait a few hours and go somewhere else.
However ... even if something DOES close at capacity, it's usually MK. MGM and Epcot have closed rarely, and I don't believe DAK ever has except during its opening week.
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