The easiest way to explain it is that regardless of Fastpass or no Fastpass, the same amount of people are going to be in the park. And at any given time ... assuming standard time allotments for eating, shopping, and non-attraction based entertainment watching (parades) ... those people are going to be in-line somewhere in the park, whether they are holding a Fastpass to some other attraction or not. So while you have a shorter line to experience the Fastpass attraction (with the Fastpass), you now have a longer line to experience every other attraction in the park (including the standby lines on all Fastpass attractions). People don't just get a Fastpass and sit on a bench for two hours until it becomes usable - they get on line somewhere else in the park. That line is now longer. Over the course of a typical day, the time spent waiting in line and the amount of attractions experienced is going to even out; most people just don't realize it.
I understand all of this, however I don't believe it is an even exchange. If 100 people get a fastpass, and all 100 of them go directly to other rides 100% of the time, then I could see an argument. But I don't believe that is the case.
And even still, for arguements sake, all 100 of those people DO ride other rides, other rides that are typically little or no wait. So now, instead of waiting 0 minutes to ride the Tomorrowland Transit Authority, I have to wait 2 minutes, or that instead of the theater for <pick any show> is now 95% full instead of 90% full. Well, I'm ok with that. Waiting short amounts of time, many times, seems better than waiting LONG amounts of time for a few rides.
If there are 10 rides, and those 10 rides can take 100 riders per hour, and the park is open for 10 hours, then the park can handle 1000 riders per hour, and can handle 10,000 riders per day.
So how does that change if there is or isn't fastpass.