But what happens if the ride with the shorter line actually has greater capacity?
Another factor to this that often gets ignored, is thinking that all capacity is the same. Or even that all attractions are the same. That they are interchangeable. It goes to reason that if you have a long line for the bathroom, you can build more bathrooms to relieve the stress, because generally bathrooms are interchangeable.
That isn't true with attractions. Building Pirates didn't make the line for Haunted Mansion go down. Building the Mine Train didn't make the line for Space Mountain shrink. Building something new doesn't make the demand for the existing attraction roster decrease across the board. Instead it pulls people out of those middle of the road experiences and causes them to wait in line, for something else. Attractions with high demand stay high, and the ones with lower demand decrease further.
And just to stay grounded, opening up a new attraction like Mine Train, which could routinely pull in a 120+ minute wait, just perpetuates the same complaints of long lines and overcrowding. Instead of complaining the wait for Space Mountain is too long, now you have people complaining the wait for Mine Train is too long.