Before you fear about the damage Quantum computers may do to BTC, fear the damage it is capable of doing to everything else that is computer/internet-related.

Just because they exist, it does not mean that they are available for hackers or negative-minded individuals yet. The entire internet/computer domain has to move towards a position where they're safe from these computers before they become widely available - nobody is that blind to let this become the thing without proper precautions. They probably pose as much of a risk to the average equipment as NASA's computers from 1995 posed for the average equipment at the time.