You'll never get that evidence; the type of people who are willing to fill out a survey are not the same type of people who don't care and are just Getting Things Done.

FWIW internally the Go team has a survey of all Googlers each year about why they are or are not using Go, but I am guessing even then getting responses about why they aren't is hard to come by.

I personally have other axes to grind that are much higher on my list (like os.File not being an interface) than generics.