You don't need a quantum computer to break encryption in order to know that it's breakable. This has already been proven (Shor's Algorthm). You also don't need quantum computers to construct stronger cryptography, immune to quantum cryptanalysis. This has also already been done as well (post-quantum cryptography).
You are likely to need quantum computers to demonstrate that they can feasibly break encryption in order to get people to switch to post-quantum cryptography.
The fact that an algorithm exists for a non-existent computer system does not meet that bar. Conventional computers can also break encryption given enough speed and power - it's just not yet feasible as with quantum computers.
