Bitcoin is not secured by blockchains, hash power, validation, decentralization, cryptography, open source or game theory - it is secured by people.

Technology is never the root of system security. Technology is a tool to help people secure what they value. Security requires people to act. A server cannot be secured by a firewall if there is no lock on the door to the server room, and a lock cannot secure the server room without a guard to monitor the door, and a guard cannot secure the door without risk of personal harm.

Bitcoin is no different, it is secured by people who place themselves at personal risk. Sharing this risk with other people is the purpose of decentralization. A centralized system requires one person to shoulder all of its risk. A decentralized system divides risk among individuals who comprise system security. Those who do not understand the value of decentralization most likely do not understand the necessary role of people in security.

Bitcoin allows people to share the personal risk of accepting and mining coin. It is only the willingness and ability of these people to resist that can prevent coercion of their nodes and co-option of their mines, and this is what actually secures Bitcoin. If people do not accept these risks there is no effective security in the money. If a great many people do so individual risk is minimized. Bitcoin is a tool, not magic.