I'm also looking any surveys that could give such information but unfortunately, I've found nothing. And if ever there is, I don't know how reliable it was knowing that we keep ourselves being anonymous to the public.

But anyway, I don't it is necessary to have those information, the exact numbers. Seeing Bitcoin still exists in the market for more than a decade, that was enough to prove that it was not scam, and many people we're succeed in kind of investment. You don't need to ask for it, or maybe one of your friend can justify it for you.