If you want to justify the claim, "future generations will look at modern medical procedure X and remark at how primitive and pointless X is," you don't need to show that all medical procedures are bunk.
Indeed, in Washington's time, not all medical procedures were bunk. Doctors helped people, sometimes. If doctors in Washinton's time never helped people, the average person would notice that, and stop calling for a doctor. Were some of those procedures harmful to patients? Sure, but not obviously so.
