And then someone sees that people are buying diapers for 10 bucks and starts selling them for 5.

Except Amazon has the the diaper production locked up, and we're back where Amazon temporarily sells them for 2 bucks while you try and get your diaper factory off the ground.

The latency between the observation of a market opportunity, and actually realizing lower prices at the consumer level, is significant. Millions of people would be charged monopolistic prices for the year it would take.

This also assumes Amazon doesn’t buy any competitor early on, such as happened with Warby Parker.

What they will see is “geeze, if we try to sell diapers Amazon will crush us”

Somewhat famously, symplectic geometry had its fair bit of foundational issues.