You do realize vinegar is a plant killer right? Too much water is a plant killer, too much sun is a plant killer. There are insecticides that kill bugs that don't have an impact on humans. Starvation probably kills far more people than Roundup. I mean I hate Monsanto as a company too but literally you could die drinking too much water. If you cannot even define how something would impact humans wouldn't the appropriate way to determine that be to perform science? Just saying we don't know so it must be bad is literally the opposite of science. People used to think tomatoes were poisonous, they weren't. Tapioca was poisonous unless cooked. You routinely ingest poisonous metals in your food. Your argument has no place in a discussion about whether something is harmful. Evidence does, once we can define the harm, if any, we can determine whether we can mitigate it, whether it's a real risk factor, and whether it's risks outweigh the benefits. You must likely drive around in a vehicle that causes more cancer and harm to the environment than Roundup. Plastic bags don't decay for generations. We have literal known problems that we give less focus than a problem that isn't even proven to exist.

You do realize vinegar (acid) is a human killer right? Too much water is a human killer, too much sun is a human killer. Science typically starts with a hypothesis and tests it. Starting with the hypothesis that roundup is dangerous for humans is logical because we already know it is not safe for other forms of life.

Vinegar is only dangerous if consumed above a safe level, same with the sun, water, etc. Which was the point you seemed to miss. Toxicity is determined not by how it interacts with plants, but the safe level for humans, which is different than plants. Your assumption that everything is automatically dangerous is not founded in anything close to science or even evidence at this point. Prove that it's dangerous, it's been the most common and least toxic herbicide for years. Wouldn't you expect more concrete evidence to turn up similar to smoking, black lung disease, mesthelioma, etc? There hasn't been a huge upsurge in cancer rates, in fact they've gone down since glyphosate has been used. There is literally no concrete evidence currently that this is dangerous to humans at concentrations currently used. Find proof then I'll believe you.

Billions of dollars for giant corporations isn't a compelling reason?

I find this a cynical or maybe a sardonic statement.

Judgment aside, there's a cost for the society which is not borne by owners of these giant corporations. These owners have a selfish interest to propagate dubious means. With "selfish" I mean the good natured selfishness in the sense of Smith's capitalism. So we as a society should strive to charge the owners with these costs, so that products like RoundUp get uneconomical to produce and sell.

Keyword: external costs

It is, indeed, a cynical statement. But it gets to the very reason why we aren't looking closely enough at the potential social costs.

As an aside, I think the conventional wisdom reading of Adam Smith, that somehow selfishness begets virtue, was not the author's intent, and is mostly just a self-serving rationalization. At the very least, it ignores the ways that selfish action can turn toxic - deception, corruption, theft, and of course externalized costs.