It has not been sustainable for 50 years because Disney, while never “cheap”, yes, used to be more accessible to the middle class, even some of the lower class (my demo growing up). It’s been proven over and over that the cost of Disney has far outpaced the inflation rates.
Your comparison, while nice, fails to take into the account the cost of living for all other expenses that are necessities now and squeezing the middle class. Yes, it’s nice if maybe Disney costs 4.25 percent instead of 3.63 (still an increase) of your baseline income. But when your grocery bill is now 8 percent instead of 5 percent, your rent 40 percent instead of 30 percent, etc it doesn’t matter. Hard to justify that expensive Disney vacation. And that’s where Disney is playing with fire. They want to cater to the upper middle class+ but if they keep pushing, they may alienate and crowd out the middle class; and what happens if and when the upper income earners stop going as often, if at all? Before you straw man this, I’m not saying Disney is going to meltdown tomorrow. But they may be in for some lean times or hard lessons if they don’t course adjust