It is difficult for me to imagine that a consensus on this would be somehow reached. I have been asking this theoretical question for a couple of months now and the community is divided almost like 50/50. Even the developers have different opinions (Pieter Wuille/Adam Back would probably prefer locking the coins, Jimmy Song favors letting them be stolen, etc.). So if the situation occurs anytime in the future there will be a huge controversy. If attacking the keys is slow the result would be probably "just" a bear market. If it is fast and huge amount of coins will flood the market I am afraid it would really endanger the existence of Bitcoin.