Comment by nadahalli

The hashing part is kind of orthogonal. The real negative outcome of quantum computers getting reliable and powerful would be that ECDSA would be broken (using Shor's algorithm variants). This would make every coin stealable by anyone with access to such a computer. Some coins would be easier to steal than others (P2PK would be the easiest - and all of Satoshi's coins are P2PK). The Bitcoin network would keep working, but coin-ownership would be meaningless.