Bogus article- they're really reaching with the whole kids cost money to raise argument. And in other news, water is wet.
My parents never would do extras. Period. Never would do dining plan, would bring their own stroller, make us carry everything because they were sure not going to spend money on a locker.
Plus, Disney is expensive, but you know what, probably cheaper than a lot of other trips. $20/day is not expensive in terms of parking. Musicals, theater, ballet, concerts- tickets are not cheap. Hotels in most places we went as kids start at $200, Disney starts at $100 on property (even if it's fort wilderness) and $50-ish off. On property comes with airport transport and parking, so even $140 is pretty decent. Food is overpriced but quick service is probably cheaper than somewhere like the Olive Garden. So in line with middle class budget, assuming that they budget to travel. Most people I know do, even if it's a week at lake a few hours away.
Disney has gone up, but so has everything else. And middle class is paying for comparatively more services these days. cell phones and wifi, for example.