You could perhaps argue that antibiotics have allowed farming practises that might not be otherwise be economical that produce more suffering for the animals.
NB I don't know if this is true or not, but it certainly seems possible.
There is an argument that the black plague was one of the driving forced behind the rise of British democracy causing a labour shortage that added fuel to the rise of the middle class.
Now I've got no idea if that is a convincing argument, but it is plausible enough to say that a counterfactual world without antibiotics might have turned out better.
Not quite. It was during feudalism, and what it established was the power of the guilds and a large rise in wages. To use the Marxist term, there was no "reserve army of the unemployed" so workers found it much easier to negotiate wages.
British democracy generally came much later, as a divide and rule proposition. The divide was between the feudalism descended aristocrats on one hand, and the merchant capitalists on the other. The franchise was extended to property owners, then poorer property owners, then all men and property owning women, then all women, then they removed multiple votes in the 60s and limited the number of hereditary peers in the 90s. The start of the process was the 19th century, whereas the plague was the 14th and then 15th.
