[throwaway2048]

This is not true, symmetric block ciphers (AES) are at worst about halved in strength.

Asymmetric crypto that depends on prime factorization or discreet logarithms (such as RSA and EDSA) are indeed fatally weakened by large enough quantum computer, but it is not a problem for other mathematically one way "hard" crypto constructs like hash trees or the McEliece cryptosystem.