"It's not clear how civilisation functions if humans become effectively useless and the only thing of value is one's ownership of wealth producing assets."
You can get a hint of this by looking at the developing countries with high income inequality. Few rich people who typically own all the countries wealth (and access to it's natural resources) while everyone else lives in a slum.
This also happens as taxes and regulations sharply increase over time.
California is a good example.
No it isn't. The median household income in California is over $100,000/year. The 20th percentile household income in California is still over $16,000/year, higher than the per-capita PPP GDP of places like Belize or Jamaica.