1) I think the issue here is that cash-like assets have a long history of being used for illicit purposes, so any decentralized alternative would need to be carefully designed to prevent abuse while still being useful and convenient. It may require a hybrid approach where certain features are centralized (e.g., identity verification) while others remain decentralized or semi-decentralized (e.g., transfers).
2) The private banks could potentially issue ecash as well, but they would need to be regulated and supervised by the government in order to ensure that they are not engaging in fraudulent activity or money laundering. This is why many countries have created central banks: to provide oversight of the financial system and prevent abuse.
3) As for your idea about using statechains primitives, it's an interesting approach but it would require significant development work and may not be practical for a wide range of users or use cases.