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.
No. The stake does not give you any incentive to promote the long term health of a PoS network. With PoS you can buy a coin, attack the network with it and then sell it. You would suffer no losses (expect potential slippage).
With a PoW coin on the other hand, you are forced to make a long term investment (spanning at least several months) in which you would suffer greatly (financially) in the event that the network was attacked