Let's say one day all of the keys from Bitcoin A are exposed. So Bitcoin A's history goes to the trashbin, but the users can, say, rollback a few days and switch into Bitcoin B, and then B would now operate independently from (the now defunct) A - assuming B survives that attack.
That now reminds me of the way Ethereum handled the infamous DAO hack. Rolling back the history to prevent big players from losing their investments. If you introduced that into Bitcoin as well, you are interfering and decreasing the decentralized aspect of the network.
If I discover your private key and steal some of your funds, you can't ask for a timeout and request a rollback. The network worked the way it was supposed to work. The person who had the private key spent the coins. It's up to the developers to keep an eye out on evolving threats and have Bitcoin switch to a quantum-resistant algorithm if needed.