Another problem is that too many of today's medical technologies are horribly marginal in their benefits. Marginal benefits have a way of smoothing out to nothing as the patient population broadens. The industry isn't approaching medicine for age-related disease in the right way. If you are actually addressing a useful mechanism, the effect won't be marginal. Science managed that shift from marginal to effective for infectious disease. There is now a pending transition for the diseases of aging - from marginal messing with the disease state to actually addressing the root causes.
Here is a phase 3 that succeeded and failed. Clearly it works. But the study design is such that they had an unexpected result:
https://www.gensight-biologics.com/2018/04/03/gensight-biolo...
They show a good effect in the treated eye, and their gene therapy also produces benefits in the untreated eye. Since they don't know how that could happen, no approval.