By doing a Google search, people are assuming that almost 1-4 Million of Bitcoin will forever be lost! If that's the case, a lot of people must have forgotten their Private key or been hacked. I've lost Bitcoin because of not exactly knowing what a private key is! You're doing the right thing by keeping your Private keys stored in an encrypted File on an external network, try to avoid keying in your private key on third-party application as much as possible!