Quantum computing "may" break what we have today, but we can always move to another chain, which would mean we would be protected anyways. The only issue is, I have no idea how the old ones would be protected, think about it, we have so many wallets with bunch of bitcoins in it, and some of them are untouched for 10+ years, could be either one issue like that guy who cashed out 80k bitcoins, come back after many years, or it could be someone who forgot their keys.

Those could be stolen, but that wouldn't really impact the market much, it would change the price if they sell it, but that is it, end of it, then we move to another level and continue like nothing happened and price would continue to soar like always. So, quantum computing won't ruin bitcoin at all.