Have you heard of the very natural Oxygen Catastrophe[1] that wiped out much of the life on Earth? Some organisms began dumping a toxic chemical (oxygen) into the atmosphere in such large quantities that it practically killed off everyone except for those weird mutants that evolved to actually breathe that dangerous toxin synthesized by the killers!
Right. Oxygen was very bad for life when it appeared. Oxygen is good for life now, that evolved in its wake. I don't want massive quantities of stuff appearing that is bad for life now.
Yeah yeah it's not 100% but it's still reasonably strong evidence. If you find something in a plant that's eaten by many things then it's probably not very toxic.
Maybe that's some evidence, but it's certainly not reasonably strong (in fact, it's very weak evidence). First, some of those "many things" would need to have a metabolism that's close to ours. Secondly, even if those unaffected organisms are similar to us, we would need to consume that molecule at similar concentrations. Most toxins have adverse effect only beyond a certain dosage. Something can be harmless (like cyanide) in it's "naturally occurring" concentrations, and positively lethal in higher doses. Besides, synthetic molecules are tested before they're approved for human consumption, and they are usually tested on animals with similar metabolism to us, and at the appropriate dose. So actually, FDA approval is a much, much, much stronger evidence for a drug's safety than it being extracted from "nature".
That idea might make you feel better, but it's not grounded in reality at all. Chemicals appearing in nature do batshit crazy things to life all the time.
If by "batshit crazy" you meant toxicity, then yes that's clearly incorrect, as others have repeatedly pointed out. Perhaps you meant chemical affinity in some general way --eg homochirality, or presence of commonly targeted functional groups?
If so, note this is the exact opposite of 'biocompatible' in the usual sense (as in 'inert'): biosynthesized molecules evolved precisely to be exceedingly good at reacting/interfering with biological processes, not the other way around. So if anything, it would be evidence they could well do batshit crazy, unintended things to life --as they often do.
