Lol!! Quantum computing is still under development and we don't yet have a realistic timeline! What we all are doing is speculation. Intel has just created a superconducting chip for such computing  and it is still being tested. If quantum computer becomes a reality, will probably cost few millions and the usage will be restricted for governmental and research related use.

Hackers won't have access to it just to launch 51% attack of crypto networks! Chill and take out such ideas from your mind!