It is true that public blockchains and other decentralised systems can have trouble meeting all three criteria of being decentralised, being secure, and being able to grow. Even though decentralised consensus mechanisms are important for keeping public blockchains decentralised and making sure they are secure, the fact that many nodes are involved in the validation process can make it hard to scale. Still, public blockchains are always changing, and new solutions are being made to deal with scalability issues while keeping them decentralised and making sure they are safe.