The discovery of penicillin by Sir Alexander Fleming probably led to the other antibiotics but as a whole I have read that anti-biotics are responsible for hundreds of millions perhaps even more, of people not dying prematurely.
That may just be the most important discovery of them all. How much longer it will be effective is anyone’s guess but I bet the come up with new ones as old ones become less effective.
Could be because in those cases they also did not give them probiotics. When antibiotics were first used they gave probiotics during and after to replenish good bacteria they knew would also die off. They don’t do that anymore.