I would also note that enough English-speaking people have tried an intensive course of meditation such as that described by the OP that even if intensive meditation had zero effect on a person, I would have expected (based on just 'raw numbers') to hear of at least one meditator who is notorious for inventing a new kind of machine, discovering a new scientific law or for some other improvement to our civilization

This is an interesting point.

I notice no change in myself as a result of meditation that I would think is likely to have decreased my lifetime potential for scientific or cultural output, but this kind of "noticing" is obviously not especially reliable.

My inclination is to think that the culture of meditation typically draws in a certain type of person, but I'm not sure that's sufficient to explain your observation (assuming your observation is true).