Living to 100 isn't the same as working to 100. Living to 100 isn't the same as being functionally aware until 100 either.
Disney at 70, 80, or 90 isn't going to be putting 40+ hours a week in the office, green lighting projects, reviewing script proposals, or vetting park rides. He might try to stick his oar in every now, but first his brother Roy, then his nephew Roy, and finally others are going to be really running things. By his seventies Walt is going to be some sort of "emeritus" who his trotted out every now and then to smile and wave. He won't be told everything, he won't be involved in day-to-day decisions, and the company will more often than not pat him on the head, give him his milk & bun, and tuck him in bed.
About the only thing I can see substantially changing is that his plan's for a Disneyland ski resort going forward.