From my inexperienced perspective, it seems that Disney will make the lines as long as guests will tolerate, regardless of how many people are in the park. Sure, each attraction has a max capacity, but do they often run at max capacity? Disney has the ability to add more staff or less, adjust the number of vehicles or tracks used for some attractions, adjust park hours, etc. All of these things could affect line length as much as the number of guests, couldn't they?
Regardless of what FP/MaxPass/ExpressPass system they modify or introduce, ride capacity seems to be a problem. But capacity is not just a matter of needing more rides. It's also a problem in part because Disney can choose to operate at less than full ride capacity. I'm sure the bean counters have worked out capacity vs. operations cost vs. profit vs. guest satisfaction vs. a million other elements. I can only imagine which are weighted more heavily. Am I wrong in my assumptions?