Not 100% sure what you mean but it depends on who makes it. A government would have better things to do with an encryption destroying quantum computer than crashing blockchains. Personally I wouldn't crash crypto coins immediately, I'd make some money off them first then wipe it out.

Also I was talking about the future. It may be impractical for centuries to make a quantum computer large enough to factor large current cryptographic keys. Last I read the difficulty of maintaining coherence has a non linear relation to the number of qubits involved.