The problem is not PoW. The fundamental problem of cryptocurrency is scalability. In my understanding, a system is called scalable if adding 1 unit of resource

The problem is not PoW. The fundamental problem of cryptocurrency is scalability. In my understanding, a system is called scalable if adding 1 unit of resource to an existing amount of resource m, improves the performance by roughly (m+1)/m. This is not a property these systems possess. The computational power of the system remains is bounded by the computational power of weakest full-node.