Seldon could not stop the Empire from falling. What the plan buys is not prevention — it is time: thirty thousand years of barbarism cut to one. The Foundation does not save the present; it shortens the dark age for those who come after.
Two trajectories of a 'civilisation level' that both collapse from the same peak, then recover as a logistic rebound — the plan sets a far higher recovery rate (it preserves the seed, so the climb is faster), the no-plan curve crawls back. The area between the curves (integrated deficit) and the horizon each takes to re-cross a recovery threshold are computed live: the plan does not lift the fall, it compresses the trough. Same collapse, shorter dark age — quantified.
The '30,000 years to 1,000' is Asimov's premise, and both curves are stylised logistic models, not a forecast of any real collapse. What is honest is the shape of the claim: intervention that changes a recovery rate changes the duration of the trough far more than its depth.