Cracking the blockchain or passphrase encryption with quantum computers, might be right around the corner. That's why we should develop CaptchaCoin. QCs won't be able to read and select correct Captchas for quite some time, yet. Besides, CaptchaCoin is automatically designed to be used in public. So, its propagation among all people will happen far more rapidly than any other crypto-coin.
Yes, and maybe the all segurity sistems, EVEN CAPTCHAS, think for a moment, if a quantum computer can hack the blockchain in only a few minutes. It will also be able to recognize the pixels to perfection in seconds, and find out what contains the captcha. Although need a database of images of captchas. I think it would be easier to hack into the site directly with the answer of the Captcha...