If the technology ever exists with the ability to crack private keys ever exists, it will probably not be used to steal any crypto, and probably not on a large scale. QC will not be able to calculate your private keys if you have never published a signed transaction with the specific private key securing your coin. This distinction may be moot if technology exists to calculate a private key within under 10 minutes.

This technology would be very valuable to whoever creates it, and its value would decrease if it were to be known to exist. If someone can use this technology in private, they can secretly decrypt certain communications and continue doing so, keeping this advantage. If the technology is public, companies and people will move to new and better encryption that QC cannot break. If someone were to use QC to steal coin, it would be obvious that someone has developed the technology and people will move to better encryption.

This is partly why I am concerned anytime I read about internet traffic getting routed through China temporarily in “error” as the Chinese government can capture the encrypted traffic and potentially decrypt it once they develop the technology to do so.