China's cheaper AI is a threat to the U.S. stock market story, which is bad news for American tech companies and retirement savers if it holds up.

• The whole U.S. market rests on the idea that American companies will earn huge profits forever because the world must use American technology.
• China is now offering AI that works nearly as well for a small fraction of the cost — as much as 7 to 12 times cheaper — and mostly giving it away.
• Other countries are backing off American AI: France dropped Palantir, Germany walked away, and Spain and Britain told their companies to stop signing deals.
• America is spending $1 trillion a year on AI, about 3% of its whole economy, while China spends far less to get similar results.

Outlook: If the world keeps choosing cheaper Chinese AI, the profits underpinning U.S. tech stocks — and the retirement accounts tied to them — could come under pressure.