If it is e.g. 2 million coins being stolen in small chunks for 10 years, the effect on price would not be so significant.
Coins are not going to be stolen in small chunks like that, they're either going to be stolen in quick succession because commercial quantum computers can in fact break SHA256, or they are not going to be stolen at all, because as it turns out, quantum computers cannot break SHA256 yet.
There are only two possible outcomes.
SHA-256 is not quantum endangered as far as I understand the topic (just a little speedup with Grover's algorithm).
We are talking here about ECC vulnerabilities (Shor's algorithm/lattice attacks).
And breaking each key can be quite a long process (= it is not the winner takes it all in "quick succession").