Well post quantum cryptography is going to evolve too. Developers aren't going to be stagnant and lets be honest, quantum computing still has a ways to go before it gets anywhere close to posing a threat to the bitcoin algorithm, or any encryption system for that matter. People expect about a decade or so. But the existing quantum technology is by far very experimental currently, and not that much of a threat to any encryption algorithm, so we are safe for now. And I expect quantum-resistant methods to be out before quantum computing can actually break these algorithms.

In fact, there are many articles COINTELEGRAPH article on quantum threats detailing approaches that are already in progress that want to mitigate the quantum threat - many aided by big corporations with a lot of funding. This is just how cryptography has always worked, someone tries to break the encryption system while others keep designing countermeasures.