No argument here. I just don’t want to count them out as a help in our overall survival.
First, Penicillin is ‘natural’, we just found a way to remake it in the laboratory, though most modern antibiotics are artificial.
But more importantly, with the newly emergent strains of antibiotic resistant bacteria, the existing class of antitbiotics might eventually become largely useless. And you might think, “Ah! The market will just make some more!” Alas, developing a new antibiotic seems to have a rapidly diminishing risk/reward and the rate of development of new antibiotics has been shrinking to zero. The trouble is that being lower lifeforms, bacteria evolve extremely rapidly, faster than the markets produce newer drugs. This might herald an entirely different mode of treatment, just like Penicillin heralded a massive shift in treatment of bacteria-induced diseases. There are in fact ‘natural’ alternatives like Bacteriophages (look them up).
As an added point, the issue is not about being natural or man-made, but about efficacy. How effective and safe is a given substance in attacking a particular disease? That needs to be established by double blind clinical testing. The trouble with what we normally refer to as ‘natural’ remedies is just that they have not been established which means we must assume that they don’t quite work. But there are lots of natural extracts that have directly been used in fighting diseases like Penicillin and Quinine (Malaria).