Another historical example that comes to mind is the Great Depression. A large fraction of banks failed in the U.S., with depositors losing all their money. In the aftermath of that disaster safeguards such as the FDIC were instituted, insuring the funds of individuals in bankrupt banks so that people no longer lost their savings when a bank went under.
Despite this extremely successful safeguard, for the rest of their lives huge numbers of people flat out refused to entrust banks with their savings. I was a financial planner for a bit back in the early 90's and heard many accounts of how older people who remembered that era were still, right up to the 90's, refusing to trust the banks or deal with them.
So the notion that a QC breach and subsequent theft of a million or millions of bitcoins would be a transient event strikes me as completely contrary to historical example on two counts. On the contrary I think it would deeply scar people's psyches, even if they rationally "knew" that the issue had been fixed with new algorithms, etc.
Yes...It probably would effect the exchange rate. However, if the controlling premises of the project were violated, then that would damage it more. The vast majority of users moved to bitcoin because of the belief that no governing organization had the power to manipulate the ledger. If it came to pass one day that ledger entries could be removed at any given time, then committing any type of resources to the security of the ledger would be meaningless. The ledger should be locked down and only those that control the private key should have the power to manipulate the funds on the ledger....period. If a hacker got my private keys, then they have the power to move my coin according to the rules. If my private keys were somehow compromised, that's my problem....They're my keys, it's my coin, so it's my responsibility....not anybody else's.
To say it bluntly, the system would be broken and meaningless if any group (government <=> community<=> third party) had the power to erase entries on the ledger. It was intended to be immutable and that is evidenced in the very first block with Satoshi's citation: "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks." That was placed there to signify the date of the first entry in a chain of immutable entries. Think about what that means. Nobody (even if it's EVERYBODY) should have the ability to prune the ledger for any reason at any time....forever. That would be an attack on the system!
Consequently, if the chain could be pruned at will, then that removes the huge potential of using the block chain to secure patents, record contracts, time lock funds, test reputation, validate identification....etc It's not just a question of how many dollars one can exchange for their bitcoin on the market at any given time....it's much larger! Contrary to popular belief, the block chain is far more valuable then it's measure in fiat!