The economy would be very dead if people only spend money on non-negotiable items. Why would one fly to Colorado if they could just go to the park near by?

Robots may not be very useful for now, but they will be at some point, and people will buy them, financing if they have to. Even if what the robots can do are not non-negotiable.

People in Europe also buy cars, even though most of them doesn't really need to.