Good point.   Today's science is tomorrow's quackery?   Understanding does evolve, at least as long as the religious fanatics do not have total power...
Seriously, leeching at one time was certainly not considered "quackery".   Maybe that is a bad example.  I think I read somewhere that leeching has made a comeback for some reason or another, or was that only a bad nightmare?
Presumably "quackery" is defined by the status quo, whether that be "truth" or not.