I can't imagine what it must have been like for people to hear about yet another "miracle drug" that cures most diseases – and then gradually realize that this one actually works.

take the word "disease" out...

and you CAN imagine it : Viagra

Ha, true! I forgot that used to not be a thing, and people were trying everything from crushed beetles to insanely hot chili peppers to crazy dangerous poisons, and still nothing really worked.

There is "antibiotics" already

I wonder why people downvote... GP said "Penicillin" as an addition to a list who had already "antibiotics" in it.

It wasn't. "Stuff that kills bacteria" has been known for a long time, including times when nobody knew what bacteria were. Not every dirty wound was fatal, there's mixes of plants, applied to wounds, that do a good job.

Penicillin was the first highly effective AND mass producible antibiotic, though.

While true, actually applying it in practice took a long time. Imagine the shock and horror when male doctors were told by midwives that washing hands and keeping things clean reduced infant mortality.