That bounty is currently at 185 billion USD. Of course, it would be pretty difficult to sell all the bitcoins without slippage, but I guess one could cash in at least several tens of millions.

Not really. BTC has no value other than its ability to be exchanged for USD. If BTC is cracked no one will trade their USD for it anymore and it will be worthless.

The first person to assume custody of a large amount of bitcoin will sell and be incredibly rich

They can siphon off a lot before other people catch on to people’s complaints and realize the best practices were followed

Yes, but anyone smart enough to run a quantum computer to solve the private keys and steal bitcoin is also likely to be smart enough to first examine the blockchain for balances that would not get complained about. Apparently, something like 20% of all BTC is just stranded inadvertently lost or destroyed keys -- starting with 'accounts' after the closely watched genesis block, but which have seen no transactions for maybe 5-8 years should net a lot of funds without being noticed.