If each Sprint is essentially to test a hypothesis (for example, Sprint 20, build a Shopping Cart feature because customers are not easily able to purchase multiple items), do you think there is enough emphasis on preparing and/or enabling the Product Owner to learn from a Sprint's results? Obviously, the Product Owner will want to determine as soon as possible whether the Shopping Cart is heading in the right direction or not.
What if...
1. End of Sprint 20 and the Shopping Cart is released