alt Hacker NewsCargo cult requires a confusion of cause and effect. Airplanes carrying cargo didn't land because there was a control tower; they landed because of prior causes that also caused the construction of a control tower.
And here, the US does not decline because of some symbolic action, but rather decline causes the action.
This confusion of cause and effect is literally a kind of magical thinking.
You're broadly right, but I think you're missing the part where perception of reality feeds back into reality, e.g. through economic effects. Demoralized Americans with less hope for the future stop caring, stop trying, and this in turn hastens that same decline.