It wouldn't necessarily invalidate any research around a link to cancer. What it shows is that any link to cancer we currently have in regards to meat eating cannot be conclusive evidence that it's deleterious to such an extent that it is an unacceptable risk to the individual or the population as a whole. If humans developed to eat meat, that doesn't support the idea that red meat causing cancer (hypothetically speaking) is reason enough to avoid it. When people come from the perspective that humans are actually supposed to eat mostly plants, or that plants are generally good for us, or that meat is empty calories, the conclusion that follows from such studies is commonly "we should eat less meat." That is wrong. The idea that plants are better for humans and that meat is both devoid of nutrition and dangerous is by no means an uncommon viewpoint.

You are absolutely right about the cost-benefit balance in the case of cancer and red meat (or really in the consumption of anything). That's why it's important for people to understand our evolutionary past, even if it might seem peripheral to the cancer link itself. If we go into subjects like this with a flawed mindset, we'll get flawed results. If it was more commonly understood that meat isn't just tasty, non-nutritious, artery clogging filler food that greedy humans only recently started doing because slaughtering animals is fun, jumping to conclusions about red meat and cancer wouldn't be nearly as popular.