The trouble is that it doesn't help anyone on average if you can't identify whether they're in the relevant subpopulation before you give people them the drug.

The trouble is that it doesn't help anyone on average if you can't identify whether they're in the relevant subpopulation before you give people them the drug. That's why the FDA doesn't approve such treatments. Indeed, drug companies slice and dice their data from early clinical trials every which way to identify the best subpopulation to run their next trial on. Their interests are entirely aligned with figuring out some population for whom their drugs work.