Standard currency!! What do you mean? Please explain to me what standard you are telling about. AFAIK, Bitcoin has a criteria to be a currency and its goal is to be an electronic currency exactly, you can see it on Bitcoin whitepaper: https://bitcoin.org/bitcoin.pdf. About private key, "safe or not" it completely depends on the user. If you can manage it correctly, then there is no problem with the security. FYI, I have stored my Bitcoin for years both on hardware and online wallets, until now there is no problem at all.