First — probably you will be interested in reading about Neven's law (https://www.quantamagazine.org/does-nevens-law-describe-quantum-computings-rise-20190618/). It is not a "law" of course but an interesting thing to keep in mind.
Second — the most powerful quantum chip for today is 128-qubit chip produced by Rigetti (https://medium.com/rigetti/the-rigetti-128-qubit-chip-and-what-it-means-for-quantum-df757d1b71ea).
Third — how many quits you need to crack a blockchain. It depends on a type of encryption, but the point is, it is 2300+ (https://en.wikipedia.org/wiki/Elliptic-curve_cryptography#Quantum_computing_attacks)
Fourth — Intel thinks we'll get 1000 quits by 2024 (https://spectrum.ieee.org/nanoclast/computing/hardware/intels-new-path-to-quantum-computing) and ECDSA will be at risk by 2027 (https://arxiv.org/pdf/1710.10377.pdf).
Fifth — implement post-quantum encryption to existing blockchains? This is a close to impossible task.
Sixth — That's what NIST says about PQC in their project (probably all of you aware of it but https://csrc.nist.gov/Projects/Post-Quantum-Cryptography):

We've mentioned a few times how quickly this field is advancing... yesterday a team at the University of New South Wales announced they have achieved a
200x speed improvement on a 2 qubit gate!

As I've mentioned before, I think that whilst a lot of work has gone into building quantum-resistant systems using classical computers, one of the best avenues of investigation is defence using quantum computers. There has been plenty of research into various methods of Quantum Key Distribution, and this research continues with approaches such as Kak’s three-stage protocol. Perhaps this will be quantum-attack-proof, or perhaps not. But the key here is that defence is actually moving faster than attack.