You know, I am surprised we don't just code into Bitcoin to shut down if faced with a need to reorg more than 5 blocks, rather than accepting an attack chain. Then all an attacker can do is temporarily DoS a chain - far less disruptive than rolling back transactions.
Shutdown and do what?
How do your resolve the fact that some % of the network believes chain A is valid and some % believes chain B is valid. Never re-org? If you re-org and the longest chain wins then you are back where you started from.