There is no system that does not have its own side effect, and blockchain has lots of side effects too, yes blockchain can be very interesting for the features it has, but still need to be controlled to an extent, if everything operates on blockchain as you said, to the operation of government, then what will then be binding the citizens legally to government for them to be able to control them?
Believe me man, you would not want to live in a world that does not have controls, so any things will go wrong even will the economy, then our prisons will also not function and you see so many people that has committed frauds become free because there is nothing that will henceforth trace their fraudulent activities. You also talked about cashiers losing their function to AI, and then these cashiers, who would provide jobs for them?
Good point. However, I believe that AI will be in charge of the world's governance, as it takes decisions based on our behavior. Laws will be enforced by smart contracts living on a single unified Blockchain network. In a decentralized world, there won't exist notaries, cashiers, bank tellers, and most other professions performed by humans today. This is because, Blockchain along with AI will be able to do everything without the need for human intervention. It's been proven that Blockchain is extremely efficient in tracking supply chains, monetary transactions, and even proving ownership of property. On the other hand, AI is proven to be autonomous and self-sufficient taking decisions on its own like a human being.
All of this, would transform our world for the better as human corruption and fraud comes to an end. The government would be operated by AI, but elected by the people as they vote within the Blockchain. Law enforcement (police) would be performed by bots controlled by AI. Using surveillance systems like cameras, and even voice recognition, AI can perform efficiently in preventing our society from breaking the law. As for prisons, it can also be managed by AI using bots. No one will be able to escape, as AI will see everything across cameras and other means.
In the case of cashiers, they will be fully automated powered by AI and Blockchain tech. Since there won't be need for human intervention, most jobs that are performed today by humans will become completely replaced by bots using said technologies. The only jobs existent in a decentralized world, would be developers/programmers, technicians and other areas related to CS and IT.
Nonetheless, a decentralized world seems to be a dream nowadays as governments have not allowed revolutionary technologies like Blockchain and AI to blossom. But, if people being to use these technologies more, then governments would have no choice but to allow them. Just my thoughts