I think that if pharma companies saw cannabis as a potential cure for cancer, we would see legal marijuana right now, at least for medical research purposes.
I would say that you are being very naive about the strength of the prohibition lobby to keep marijuana a schedule one drug. Possibly the only lobby stronger than the pharmaceutical one. Personally I expect any significant research in this field to come from overseas. Especially a country that isn't so financially tied to keeping marijuana illegal. The major thing holding most of those countries back is their small population base from which to derive funding and/or a subject pool.
Consider the aspirin example. Aspirin is derived from the bark of a willow tree. If aspirin were discovered today it wouldn't make it out of the lab many contend. It just doesn't test well due to some potentially bad side effects. However, people love the stuff. For many people the anti-inflammatory benefits out weigh the gastro risks and they educate themselves on taking it responsibly. But back to our fantasy world and lets assume pharmaceuticals figured out that aspirin worked by inhibiting cyclooxegenase enzymes, they would surely try to break the vascular effects of the drug apart from its anti-inflammatory effects. They would try to find new compounds that selectively inhibited only one of the enzyme subtypes. They would, in other words, produce Vioxx. So which is better low dose aspirin a natural compound or Vioxx as an anti-inflammatory? Hint Vioxx was pulled from the market but go ahead better living through chemistry guess which one.
Many things have been observable throughout history long before there were scientific studies to confirm the original observations. I willing to accept that it is possible that marijuana can shrink cancer tumors or inhibit their growth. But I am also willing to acknowledge there simply hasn't been enough unfettered scientific research in this area.
The thing I find most alarming is due to government funding and regulation scientific study has become politicized especially in this country. And this politicizing of science has hurt the confidence in scientific research as a whole. I honestly think the question would have been definitively answered long ago if government wasn't in the way. That means if marijuana was decriminalized, labs were not government funded and the drug industry wasn't so heavily regulated.