Is it possible that all current blockchain-based cryptocurrencies (like Bitcoin) can be hacked in just a matter of seconds? It indeed seems possible, based on the statements made by Dr. Joseph Liu, Associate Professor at Monash University.
Dr. Lius qualifications add allot of weight to his statements. He is the Head Researcher at Monash Blockchain Research Lab, the Chief Scientist of HCash and he is also the co-creator of the Moreno cryptocurrency. In other words, his statements about cryptocurrency development should be taken seriously...
https://www.bitcrypt.co/every-cryptocurrency-hacked-in-seconds/Quantum computing could arguable do something like that, but right now we don't know how to use it effectively for normal computing, let alone cryptographic cracking. Quantum computing would basically be breaking any kind of encryption anyway, so there are much more concerning aspects to it than what would happen to Bitcoin and cryptos.
As for what can be done about it, well, we need to find a way to get consumer quantum processors so that we can see what it is capable of. Then we think about what it can't do and exploit that as the basis for a new form of cryptography, and, if necessary, apply it to Bitcoin.