True. But for the highest-grade nasties, where median life expectancy is unfortunately short and progression near-universal, you don't need much signal to get above the noise.
Anyone surviving more than a handful of years with something like that is an outlier such as to merit a full work-back, and at that point it's no longer bro science.
I think that's partly a survivor (the disease surviving, not the patient) bias effect.
Things that could be solved by random discoveries are no longer considered the highest-grade nasties. There were a lot more intractably fatal conditions in 1870 than there are today.
So the likelihood of there being answers that could have been randomly discovered by medics with 1870 or 1920 levels of knowledge is tiny. At the same time, the sum of human knowledge has expanded so rapidly since then, it's not impossible for stuff to get missed.
Conversely, those are also the least likely to be solved by random trial and error.
Those people largely just die, no matter what you do — that’s what makes it a “highest-grade nasty”.