That wouldn't help. People's expectations for standard of living would rise, and both parents would continue working just to keep up with the Joneses.
Furthermore, in most families the main reason for needing dual incomes is to pay for housing. Increasing productivity does nothing for affordable housing. Real estate prices tend to rise right along with average incomes.
The one-parent-quitting description is to show that the same amount of output/production/work can happen with half the people if they are twice as productive. This is to show that increased productivity (more product, same amount of work) is more important than jobs.
Increased productivity gives more of everything for everyone. We all know this to be true - if you spend all day napping in the sun, you get nothing achieved. If you spend all day working, you can get many things achieved.
Furthermore, in most families the main reason for needing dual incomes is to pay for housing. Increasing productivity does nothing for affordable housing. Real estate prices tend to rise right along with average incomes.