If 25% of the coins are vulnerable, what has to be done is to educate people that they should not re-use addresses. Satoshi's coins and other "lost coins" where this is perhaps not possible are only 5%. And these 5% won't be cracked instantly by quantum computing, it will take time. So it's not that we'll have 1 million BTC of new market orders on exchanges in one single day (which could be disruptive).
In a way satoshi's stash and other old "lost coins" will serve as a canary given that they will likely be the first target. That is to say, even with this milestone reached QC will in all likelyhood still take years if not a decade or so to reach the point where it can feasibly snatch coins mid-transaction.