The problem is that they currently are running at full capacity, just doesn't feel like it cause more than half is dedicated to Lightning Lane it feels like.
They just haven't added capacity to the parks that can be utilized. Epcot has enough capacity, problem is that nobody is riding Imagination/Mission Space or sitting inside the Imagination/Awesome Planet theaters.
For a park like Epcot, it needs "new" stuff to replace some of the lesser-used attractions. It's insane to think that Figment, Imagination Theater, Awesome Planet, and Mission Space all combine for nearly 6-7K theoretical hourly ride capacity that mostly goes unused.
For parks like DAK/DHS, it just needs more new stuff. DAK is built to accommodate large crowds (it has the dining capacity for it as well), it just can't keep people there for long.
MK, needs more "space" but not necessarily more attractions. I think MK really only needs to pull a Dumbo with Peter Pan to help capacity, and get Tomorrowland refreshed. The Main Street bypass with the theater would have done wonders to the park as well. Call me crazy, but I don't think MK needs that "Behind Big Thunder" thing as much as DHS/DAK need investment.