The quantum computer is one of the most incomprehensible machines ever built to this point. Imagine running the same calculation in multiple dimensions/universes at the same time and then observing those solutions so they collapse back in to this dimension/universe and then combining the solutions for all possible answers. The more powerful these computers get, the faster they will be able to solve these sha256 problems. So it's something to be considered but probably not concerned as by the time there are enough Qubits used, we will have moved on from Blockchain to Hashgraph or something that we haven't thought of yet.

IMHO.