This question is nonsensical. Let's say Amazon leaves the EU. What does that have to do with the Fediverse? If all US social media companies left, people will find other forums to engage in. This could be the Fediverse, could be something else that hasn't been created yet.
In the long run, I think we will all end up on a version of federated social media. The corporate silos will lose their appeal, regardless of where they're based. Whether this future federated service will be based on Activity Pub or AT or whatever protocol no one knows right now.
Trump's tremendous social media platform truth dot barf runs on Activity Pub, they just don't federate with anyone by default. It's like they don't want dissenting views on there. Weird.