Also, Walt became ill right around the time that the biggest decisions for Mineral King were being brought to the table; I'm pretty sure that Roy knew that Walt was fading, and he may have deliberately let Mineral King die, since the Florida Project was such a huge undertaking, and he suspected that it might all fall on him sooner rather than later.
Technically, an indoor park was tried; that was DisneyQuest Chicago. The truth is that parks with the size and theming of Disney or Universal need sunshine; they need viable year-round operations in a place people WANT to travel to in cold weather. Locals alone can't support a park if there is a true winter off-season for tourism in the area as a whole.