Irene, what you describe is what I once thought would happen. Generally speaking we need a planetary government with more teeth than the UN but things are changing very quickly now.
We no longer need vast numbers of bureaucrats to manage hundreds of millions of people. We no longer are limited to human mediated information flows. Our future is automated administration with little or no politics as we know it.
People have too little interest in abstract questions or process efficiency. Unless there is a personal connection people don’t care about the planet. We need to change that but it is not, I think now, going to work as human political choices.
Politics beyond an immediate local community is to difficult for most people. Instead they accept memes and slogans whether valid or not. At the community level where they live and work and talk to people they have positions on things that affect them. Roads, transportation, buildings, community services. They can actually make decisions on those things.
Talk to them about the planet and they have no real grasp of what that means. This is, in effect, the position taken by the Chinese government that decisions need to follow rules with results that benefit the majority of people and that is best done without getting entangled in too much direct popular involvement.
The way this will go, I think, is with algorithmic models based on local populations. We (our AI/ML helpers) can tell us what people think and want everything they say and do publicly and with public entities. If it’s important people can be asked to confirm the model assumption.
Based on this we can eliminate most layers of government as irrelevant. What would they do?
While we are not there yet we are getting close. Human audited administrative models constantly updated for the population they represent can provide greater sensitivity to the process and services provided to all and specialized services provided to some.
This would be based on planetary agreed rights and minimum standards of wellbeing. These could be selectively added to at each urban (Metropolis) level but could not be reduced. Resources above those needed for minimum wellbeing could be distributed, invested in various ways, or used to fund private ventures. Those would be the decisions most interesting to people for their communities.