A lot of medical practice is based on presumptions

Some time ago I had a radial head fracture (a fracture of the end of the radius bone that's closest to the elbow). The traumatologist said that in the past, and for the longest time, the arm would be completely immobilized for weeks for such a fracture, but nowadays that's not recommended because not only does it not speed up the recovery, but moreover it may cause a permanent decrease in the range of motion of the elbow. Instead, it's recommended to just leave the arm as it is and to periodically (and carefully) make motions with the arm in order to maintain the mobility of the elbow.

Turns out that the practice of immobilizing the arm for several weeks for this particular type of fracture existed solely based on presumptions, not clinical trials and studies. And this had been going on for surprisingly long (decades and decades) without any corroboration. It was essentially medical tradition based on baseless presumptions.

The scary thing is that this isn't, by far, the only example. There are many examples of medical practice that was very common in the past, and for the longest time, that has turned out to be incorrect, detrimental or, even in the best case scenarios, just useless.

For example, nowadays it's not recommended to artificially lower fever when you get some kind of viral or bacterial infection. Turns out that the fever happens for a good reason, and artificially lowering it only hinders the reason why the body causes it in the first place (essentially, a higher body temperature slows down the spread of infection and helps the body's immune system to fight it off. Lowering the body temperature only helps the infection and hinders the immune system, thus prolonging the infection and making it worse.) Fever is artificially lowered only if it gets dangerously high (which is rare) or if there are other reasons to do so, but for an otherwise healthy person it's not recommended.

Examples of this and other detrimental traditional medical practices listed eg. here.

I actually find it scary how much of medical practice and medical wisdom is based on nothing but presumptions and assumptions. And, especially, how long such practices have been going on unchallenged, before they have been finally actually clinically studied and tested in order to see if they are actually sound. In some cases such practice has been going on for decades and decades before the medical community finally got around to actually verify that it actually works.

One has to wonder how much of current medical practice and knowledge is still based on assumptions and is actually detrimental rather than beneficial, because they have yet to actually corroborate the efficacy.