If by “crypto,” the grandparent meant “cryptography,” this is not true. Most widely-deployed asymmetric/public-key primitives (e.g., RSA, elliptic curve cryptography (ECC), etc.) are quite fragile against an adversary with a cryptographically-relevant quantum computer (CRQC). To clarify how fragile, the general consensus/state-of-the-art as far as I am aware, is that Shor's algorithm (which breaks asymmetric primitives) requires about 2x the number of perfect, logical qubits as the RSA key-size (e.g., ~4000 qubits for factoring RSA 2048); however, because none of our qubit designs have a low enough error rate, you need about 1000 qubits to simulate/error-correct for a single logical qubit—so, currently, it's expected you would need around 4_000_000 physical qubits to factor RSA-2048. Post-quantum cryptography (PQC) is specifically the subset of cryptography that is designed to withstand attacks from quantum-enabled adversaries; it is still being actively designed, studied, standardized, implemented, and deployed.
If instead, the reference was to “cryptocurrencies,” most cryptocurrencies I am aware of depend on non-PQ constructions, and fall into the same buckets as RSA and ECC. Some systems, like Bitcoin, are in significant danger without large overhauls—if a practical CRQC is actually realized. There are efforts underway throughout the cryptocurrency communities to try to prepare for such an eventuality, but to my knowledge, none of them have major adoption yet.
As a final note on investment advice: I don't give out investment advice. :)
As I've mentioned to another commenter, Bitcoin relies only on the existence of an arbitrary DSA. Quantum computing-resistant DSAs have been known since the 1970s. I reckon that swapping out Bitcoin's current DSA with a quantum-resistant one would not count as a major overhaul. https://news.ycombinator.com/item?id=43113682
I'm not sure about other cryptocoins, but Bitcoin does not use encryption, it only uses authentication, which requires a DSA (Digital Signature Algorithms). Bitcoin's current DSA would in fact be broken by a cryptographically-relevant quantum computer (CRQC). However, there are DSAs - like Lamport signatures and Merkle signatures - known since the 1970s, whose security depends only on the existence of ANY secure hash function. There is no known way to break any widely used hash function using quantum computers. So I reckon that the only change to Bitcoin would be to swap out the current DSA for a different one.
I'm not sure about the downsides of quantum-resistant DSAs.
