A classic counter example is how long it took for hand hygiene to be accepted as an important aspect of medical practice. The cost was just about zero. The benefit: thousands of human lives saved. Still took a hell of a long time.
Another example: scurvy. Known to be cured by lemons since 1601; took 200 years and the better part of two million lives for this news to be properly disseminated.[1]
Still looks partially like a cost/benefit thing (even though there were indeed many doctors not believing lemons were the cure and instead prescribing "fizzy drinks" with sulpheric acid).
Another example would be anti-vaxxers today. Here too, though, we have a special case. The technology was so successful that people forgot how bad of a problem it solved.
Interestingly, in these cases my statement about lack of success implying lack of benefit is wrong, but these are the very cases where the benefit is easily proven empirically.