I think there will always be the appeal, but the real issue is how sustainable it will be to host families of all ages in 100 years, unless they can figure out ways to house almost everything they offer indoors to control the climate.
As a Florida native, I've felt first hand the dramatic escalation in heat & humidity over the past decades. A Florida summer today feels nothing like it did when WDW opened in the early '70s or when I was growing up. It was always hot, sure, but manageable.
Today's heat extremes prohibit most people from tolerating multiple hours outside during majority of the year, unless at the beach or pool. And temp trends are rising, not declining.
Theme parks that will survive & operate year-round will likely adapt a version of indoor water parks up north. They'll have to control the climate for guests far more than now.