IMO, it’s not a serious concern. Humanity may eventually invent this technology, but there would be signs, and we could take steps.

The first sign that anyone had a functional, ECDSA-reversing, quantum computer is movement of super old coins. Like Satoshi’s coins. These represent a huge bounty for would-be quantum pirates.

If we were concerned about this possibility, we could softfork in additional, quantum-resistant signature schemes and start to move coins to them.