Thank you for youre answers! 
But did you all notice, that bitcoin.de isnt "a normal Onlinewallet", its an Onlinewallet with a lot of security mechanisms. This Onlinewallet even has got an own forum and they claim to be one of the safest Onlinewallets worldwide.
You're definitely still hugely overestimating it. But did you all notice, that bitcoin.de isnt "a normal Onlinewallet", its an Onlinewallet with a lot of security mechanisms. This Onlinewallet even has got an own forum and they claim to be one of the safest Onlinewallets worldwide.So in conclusion the safety of the Onlinewallets might be a little bit lower than I mentioned, but its not close to 0%! For example bitcoin.de never got hacked and the site is over 5 years old!
Oh boy this has gotta be the worst argument ever concerning security."Hackers can't hack X exchange/wallet because it's already X years old and never been hacked!"
Guess what, even Coinbase is vulnerable to hackers. Remember, MtGox and Bitfinex was once also one of those supposedly "very secure" exchanges.