People invented it because they were LOOKING for antibiotics explicitly. Fleming had previously found interferon, had cultivated slides where he could see growth irregularities very well, etc. The claim of fortuitous discovery is basically false modesty (see "Discovering" by Robert Root-Bernstein).

Even if we prefer to frame the reference class that way, we can instead note that anybody who predicted that things would remain the way they are (in any of the above categories) would have been wrong. People making that prediction in the last century have been wrong with increasing speed. As Eliezer put it, "beliefs that the future will be just like the past" have a zero success rate.

Perhaps the inventions listed above suggest that it's unwise to assign 0% chance to anything on the basis of present nonexistence, even if you could construct a reference class that has that success rate.

Either way, people who predicted that human life would be lengthened considerably, that humanity would fundamentally change in structure, or that some people would interact with beings that appear nigh-omnipotent have all been right with some non-zero success rate, and there's no particular reason to reject those data.