Cures can be forgotten without a preceding golden age. It’s always one step forward, two steps back with pre-modern medicine, but that one step forward did indeed happen, repeatedly.

There were no reliable ancient cures, because we were unable to create any before we had enough scientific knowledge about our bodies to consistently replicate their effects.

Many (well, some) herbal remedies etc. did and do actually work, it was just more difficult to match them to the illnesses they were actually useful against and they were less effective than our contemporary pills. e.g. they definitely had painkillers, disinfectants, emetics and laxatives. Calling them “cures” might be a bit much compared to e.g. antibiotics, but these types of things definitely still have medicinal use.