Blockchain technology holds the promise of transforming our financial system, but a key question lingers regarding whether this technology can ensure secure settlement. We develop an equilibrium model to study that question with regard to the most prominent blockchain type, a Proof-of-Stake (PoS) blockchain. We demonstrate that blockchain settlement security increases with the economic productivity generated by the blockchain. Moreover, we show that blockchain settlement can be secured against arbitrarily large disruption incentives so long as the blockchain generates sufficient economic value.