A quantum computer can crack ethereum and bitcoin in 1 hour. So now there are many forks that are more protected
How can you say that a quantum computers could crack those cryptocurrencies even though they still doesn't exist yet. Stop making claims unless it is true. People who holds bitcoin will go panic selling making the bitcoin price goes down again.
There are many types of hacking. If the trojanhorse passes in your computer your password, id and username will documented to reached to hackers desk. In these kind of advance ethical hacking and power with the super computers or quantum means it is easy possible to hack the wallet.
If you want secure your wallet, please do not have your funds in online wallet and keep your private key and wallet.dat file much safer in any place.