1. That's not an argument unless the evidence for these payoffs is so huge as to dwarf the payoffs of 1000 smaller experiments. There is no evidence of this.

2. There is no world in which this applies to particle physics at this point, especially using radio frequency particle collider tech. This is known physics and there are no mysteries in the regime the FCC would reach.

Do you have evidence that "1000 smaller experiments" would give payoffs?

And how do you measure payoffs? With how much money you get in return? Should scientific research expect this?

Payoffs have many forms, the most important for pure research being "advancement of knowledge". We have nearly zero expectation of knowledge advancement from yet another radio frequency collider.

Then the mystery is how the CERN "raised" those $1B. Maybe they have an amazing PR department? Or maybe the project is going to be such a huge success that they are acting from the future [1]?

[1] https://en.wikipedia.org/wiki/Steins;Gate_(TV_series)

CERN and large construction projects like the FCC employ tens of thousands of physicists and engineers across decades. It's hard to convince someone of something when their livelihood depends on them not believing it.