Quantum computing is a threat because if it became mainstream usable today it could render the entire password based login system hackable in a flash

Oh not just passwords and logins, all kinds of cryptography, including any encryption, certification or validation based on them, would potentially become trivial. Again, it’s hard to say exactly what the outcome would be because we don’t actually have functioning quantum computers in a practical sense. But if we did, it is plausible to imagine that all practical security measures of any kind on the Internet, or anything using DRM technology, or anything wireless, ranging from top secret military and satellite communications, to industrial SCADA systems, to cellphone networks would instantly become a wide open security vulnerability with open access to anybody with such a computer.

It would almost certainly be catastrophic. A digital dark age. Everything would have to be shut down or disconnected for immediate, urgent redesign. We’d need to do almost a total reset from scratch of the entire electronic world, even the power grid itself. Even assuming we quickly develop a new quantum-resistant or quantum-powered security infrastructure, it will quite likely be vastly more limiting than our current systems and will require massive compromises in the ways we are used to designing and interacting with technology.