Here's something to consider. Even at today's insane prices, Disney managed to fill 8.3M, yes that's million, room nights. Just to throw out a number, if you assume an average stay is 7 nights, that's 1.2M families who coughed up Disney's prices.
We've read on these threads plenty of posts from people ranting about how much they loved MyMagic+. That's what Disney is hoping for.
Disney is hoping they can find a couple of hundred thousand more families to switch from offsite and book their 7-night stays onsite instead. 200K more onsite families corresponds to 1.4M room nights. In 2013, Per Room Guest Spending (PRGS) was $267. $267 X 1.4M = $374M. That's just in one year at current PRGS (which can be expected to rise).
An occupied room at a hotel that's already running at over a 50% occupancy rate represents almost pure profit. Last year, Disney's domestic hotels ran at 79%.
Again, Disney only has to influence a few hundred thousand families. They don't need more visitors, only more visitors staying onsite and (per the original budget), MyMagic+ becomes a big financial success.