Because the majority of guests are not the ones staying at the Deluxes or DVC and buying LL or VIP Tours. Do you think it is fully sustainable to depend on the expectation that fewer guests will spend more to cover the loss in number of average guests?Disney obviously has to continue to offer a product that is appealing to those with the money to enjoy it and can certainly screw that up. Maybe they already are. However, I don't see how you can make a clear case that Disney needs to continue focusing on a consumer group that is declining both in terms of relative size and income versus another that is only likely to continue becoming more lucrative.
Thinking back to the article that sparked this thread, you can clearly see that Disney still provides avenues for those in the middle income bracket to experience WDW. Presumably, with greater capacity they can also provide more avenues to appeal to this audience. If you were running the company, though, would you really look at the numbers and say "we need to focus on this demographic that is declining in almost every respect"?
I think what you're getting wrapped up in the income brackets. Disney isn't pursuing the Upper Income Bracket, they're pursuing the 1 and 2%. A family of 4 with a household income of $180K does not have the same disposable income as a family of 4 with a household income of $500K. Disney wants them more than anything, and they don't exist en masse.