I'm no WDW expert, but didn't FOTLK start as a temporary show in a temporary land scraped together from some abandoned parade parts?
Nope, it didn't. At least, not in the way you're thinking.
All shows at Disney are, technically, "temporary". Nothing is built to last to the end of time. When FoLK was developed, it was developed to be a standard stage show with a standard 4-5 year shelf life. It was put into Camp Minnie-Mickey -- which was a viable and, at the time, permanent addition to the park. The wooden structure of the show venue was reminiscent of the outdoor amphitheatres you see in camps in the Adirondacks, which were camps that Michael Eisner attended as a kid and carried around a lot of nostalgia for. Hence ... the theme.
The floats for FoLK utilized existing float bases and also brought in elements of set pieces that had been built for the grand opening of the park. But it's not like they took left over MK parade floats, popped a lion on them, and turned them into FoLK elements. Those pieces -- and the puppets on them -- were built for the show using some recycled elements, as Disney tends to do. But each one was built specifically for the show.
A few years after FoLK opened, it was so popular that the existing theater was enclosed so that shows weren't being lost to rain, heat or other elements and so that the lighting, audio and effects could be controlled more consistently. This was also done to make it a more "permanent" venue and to give FoLK a long-term home.
When Avatar was announced, and the real estate for it identified, there wasn't even a question as to whether FoLK would continue. If it was meant to be a "temporary" show, that would have been the time to close it. Instead, the new Harambe Theatre was built, and Disney used that as an opportunity to give the show the facility it deserved -- both on-stage and backstage. If anything had been cobbled together at the old venue, it was the backstage areas. Trailers, tents, part of a parking lot, a Quonset hut, and an off-site rehearsal facility were replaced by a series of offices, dressing rooms, a green room, cast areas, rehearsal facilities, stilt & puppet areas, and other things designed and built specifically for the show.
Which would indicate to me that FoLK isn't going anywhere any time soon. It's been around for 16 years. I'll be pretty darn surprised if this "Frozen" summer event is still being done 16 years from now.
YMMV