What Is the Main Defense Mechanism against a 51% Attack in Proof-of-Work Systems?

The main defense mechanism against a 51% attack is the immense economic cost required to acquire and maintain the necessary hashrate. The high capital expenditure and operational costs (electricity, hardware) act as a deterrent.

Additionally, the decentralized nature of mining pools and the community's ability to coordinate a hard fork to change the mining algorithm can mitigate the attack's long-term success.