I admire your optimism in epidemiology. In point of fact, though, we have a rough natural experiment in the form of a food that doesn't cure a disease, but rather makes half of all drugs worse. That's very valuable knowledge, and under ideal epistemic conditions it might have been discovered within a few years of organized drug discovery as such. Yet was not widely known until the 1990s. So that's a failed positive control, which suggests that our practical capabilities to detect these kinds of effects are limited. Understandably so, given that there is no general requirement for dietary logging in clinical trials.

That said, "a food that cures cancer when eaten" is not the bar for experimentation. More realistic might be something like "a dietary or behavioral protocol that, in some way, ameliorates this or that indication".

For organisms with our body plan, "a cure for cancer" is like talking about "a cure for defection". But clearly there's "stuff that is efficacious against particular instances of cancer", a lot of which we found through techniques like natural product screens, i.e. "just trying stuff", rather than via rational drug design.

It is indeed somewhat surprizing why it took so long to figure out grapefruit juice (and it was by accident during the study of the effects of coffee on a drug). Some of the early drugs that interacted with grapefruit juice where short-term agents like antibiotics, and others like channel blockers didnt have obvious detrimental toxicity though they might have had effects that were above the range of typical human variability. Powerful statins became household items a bit later, and all the cancer drugs were invented after this association was already known.

I think there is a lot of efficacy left on the table with modern studies, clinical trials, and drug development. Most of the work is looking for treatments that work for the majority of the population. If a treatment was effective than standard care for 10% of people, it would be discarded.

It is more economical to start over looking for something that works for 90% of people.

Drugs an treatments aren't evaluated and discarded when proven worthless. The bottleneck is the number of treatments in development, so they are discarded when something else has a better ROI.