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.
What are you on about? What part of: IF YOU DID IT ON YOUR PC IT IS PROBABLY COMPROMISED don't you understand?!
If it is a PC that was never connected to the internet, it is not. Unless you want to use magic again, which is unfair.
Furthermore, I just wanted to show, that you can not really predict how private keys are generated. You can't even predict, when a private key/public address was generated, since I can generate one today and use it in 10 years and you wouldn't know.