Stake isn't capital. You can base PoS on tokens like travel points. Capital is fixed value hardware or natural resources. Bulldozers, ASICS, oil, and licenses are examples of capital. PoS tokens are speculative vehicles that may or may not have value except as a security token like a Verisign cookie.
Since you consider licenses (intellectual property, not just physical property) an example of capital, think of the stake as a license that you invest into.