He didn't? He repeated that exposed queue in 1961 with the Flying Saucers, and again in 1966 with it's a small world. And although he passed away in late '66, the plans were complete for the early 1967 debut of Pirates chain switchbacks along the side of the building, and 1969's plain switchbacks behind the house for the Haunted Mansion.
Walt's stuff for the 1964-65 World's Fair was no different, even though he had the deep pockets of American Industry to pay for stuff. People waited in long switchbacks lines out in the New York heat and humidity each summer in metal barricade switchbacks...
Before they finally and mercifully snaked their way inside the pavilion and got loaded into a new Ford, Mercury or Lincoln convertible for their ride through Walt Disney's Magic Skyway.
The Carousel of Progress, a beautiful building and pavilion with many exhibits besides the main revolving theater show, was also surrounded by plain switchbacks.
An argument could be made that the August, 1967 debut of Monsanto's Adventure Thru Inner Space was the first fully themed queue, but that was an aberration for Walt's era.