Practical quantum cryptanalysis is already the end of Bitcoin, which relies on ECC algorithms that will fall to QC.
(Maybe you're making the broader statement, that if QC turns out to make all forms of public key crypto insecure, regardless of the hard problem they're based on, then it won't even be possible to design a working alternative to Bitcoin.)
Addresses that have been sent from will be vulnerable because their public key will be displayed in the outgoing transaction. One-use addresses will not be vulnerable, as their public keys are indeed stored only as hashes in the blockchain, but the algorithm is not only RIPE, but something like SHA256(RIPEMD-160(SHA256(Public_Key))). I dont know about RIPE, but sha256 should be QC resistant. Thus if you do not reuse addresses you should be safe against QC.
(Maybe you're making the broader statement, that if QC turns out to make all forms of public key crypto insecure, regardless of the hard problem they're based on, then it won't even be possible to design a working alternative to Bitcoin.)