Well, all of it was due to the lack of medical knowledge. The bubonic plague hardly kills anyone anymore. The point the parent is making is that we shouldn't compare the societal effects of COVID19 today with that of the bubonic plague when it was at its worst.
But his conclusion seems to have been: we need to worry about what happens if a disease like the black plague hits again: which, well, no we don't. If we need to worry about a future pandemic its one which is quite different the plauge and thus one we are defenseless against.
I believe the point was that we need to worry about a pandemic that is as bad as the black plague was, not that we just need to worry about that disease specifically. The OP even mentioned bird flus as one class that could cause similar devastation.
Why do you think that? We could work to improve our ability to rapidly develop vaccines and stockpile PPE. Those two things would go a long way. Analyzing our supply chains and finding critical points of failure would be another conceivable step.
