[mlyle]

That's close to what I tried to say.

Perhaps you prefer: A company must think it's likely that they'll have a good return on all development costs, not just the costs of drugs that happen to be successful, to continue to invest.

> if profits are high, the industry expands to try more kinds of drugs, stopping when the attempts on the margin are just profitable enough (on average).

Of course, something like pharmaceutical products, with exclusive sales of specific products, few sellers, strategic conduct relative to other industries (insurers), and heavy regulatory influence is not guaranteed to converge to normal profit.