In case of productivity surplus, exponential demand can be easily created by government: e.g. they print N trillions dollars, and say we spend it on some (hopefully useful) mega project(s): fusion, carbon capture, give nice home to everyone, fully verifiable, glitch free, hyper optimized software, etc. It will immediately create lots of demand, supply chains, customers, etc.

You can see current AI hype as example, and its government didn't step in and printed N trillions yet.