The beginning of the end almost always starts with some out-of-their-element executive saying "We have to protect our business model!" instead of "We have to do what's right for our customer!"
Amazon banning agents is the first opportunity I've seen since Amazon was founded for a startup to create an Amazon competitor. People will want agents to buy stuff for them. It will be one of the main use cases. And they won't want to use some Amazon-supplied agent to do it.