I think it depends on what Disney sees as the life of WDW as a resort destination. By selling timeshares, Disney is making a committment to the resorts through 2042. In 2042, MK will be 71 years old (1971, right?).
I don't know that anyone has any idea what the life of a theme park is - particularly this style of resort destination. And Disney has been doing things that show an interest in maintaining this life for the long term (opening new themeparks). But everything does have a life. Will Disney be around in 2050, I think so. Will they be around in 2075, 2100, 2150 or 2250?
My guess is that when they extend, the end date will be moving - a 25 year contract regardless of whether you join this year or next....with an EOL on the resort (and an option to switch resorts, allowing them to tear down and rebuild)