The average family rarely, if ever, purchases one-day/one-park tickets. It's surely not something Disney wants to provide significant incentive for, prefering to encourage guests to stay longer. Beyond that, I think the average family can afford the cost of admission. It just requires a bit of creative prioritization, deciding to take a WDW vacation instead of, say, buying an HDTV and wearing winter coats for one-more-year, for example. Many families won't see the value in making those sacrifices for WDW, though, and they'll choose to vacation elsewhere. It's a delicate balance.