Well - it's a good point. For instance at DLR - they have a known problem of having too many AP holders, and a few years back drastically raised the price with the purpose of trying to reduce the # of AP holders. As you say, if you charge $100 per person and get 25,000 people at a party, or you charge $80 and get 31,250 people that's the same income - you still get $2.5 million per party.
The problem is - that's not the only piece of income to consider. Will those 25,000 spend as much in merchandise and food as the 31,250 people did? The answer is likely "No they won't". In fact the people paying $100 are more likely to spend LESS than the people spending $80. I think somewhere in this thread there was an article about that, that the problem Disney is seeing with rising rates is that while attendance has continued to rise, guest spending has been flat. So the problem is the family that was spending $5000 on a vacation and $1500 on merchandise is now still spending $5000 on vacation, but only $750 on merchandise. Or bringing more of their own food. Whatever the reason - it means that raising ticket prices isn't helping. And if they raise it enough that people stop coming...well then you have a problem.