Let's compare the standard sovereign security model that exists on current proof-of-work (PoW) chains to that of the shared security of Polkadot. Chains that are secured by their own security model like Bitcoin, Zcash, Ethereum, and their derivatives all must bootstrap their own independent network of miners and maintain a competitive portion of honest hashing power. Since mining is becoming a larger industry that increasingly centralizes on key players, it is becoming more real that a single actor may control enough hash power to attack a chain.