Im only showing you a fraction of what was there, just shy of 200 massives buildings, look at them and tell me if they look "temporary" to you.
By present standards, no.
But they do look like the Hollywood movie sets of the time, like from "period films" set in earlier heights of civilization (Egypt, Rome, Greece, etc). Check out the end of
Intolerance, for example. Fake buildings back then were drastically "overbuilt" by today's standards, because, to oversimplify a bit, nothing was expensive.
(A late example: the wood- and plaster-work in David Lynch's
Dune. He wanted that old Hollywood look so he had the sets built in Mexico, to cheap out on materials and labor. It looks like it cost a billion dollars, and it couldn't be done today at any price. When they were done shooting, they broke it all up and threw it away.)
People are right to sense civilizational decline in those World's Fair pictures, but it's the mundane economic kind. We got ripped off, all our stuff replaced with garbage, and we got billed for it (with interest).