OP is perfect example of people still not understanding every tiny thing in BTC is recorded.
The resources used to attempt and cheat the system will never be worth it ever.
Believe me, I understand the idea of a distributed, peer-verified ledger. Perhaps you still do not understand that "being worth it" SHOULD NOT MATTER. The system is built to be mathematically strong, not strong due to someone's predictions regarding human nature and incentives.
It isn't worth doing something that will just be erased within the system itself.
This is the first time in history where thieves/bankers can't steal without being publicly seen throughout the whole system.
A fork would terminate any massive attempts, and it would have to be massive for the resources used to be almost worth it, but again it will never be worth it period.