So, how do you calculate the amount of private keys from people who role dices to make them. How do you count the people who let their cat run over their keyboard to generate it? There are a lot of possibilities to create private keys, without even software.
Your last point is just stupid: Used Bitcoin addresses and generated private keys are just not the same thing.
So, yes you can collect some data, and make a guess, but it wouldn't be a good one.