>The problem microservices try (tried?) to solve isn't about namespaces, it is about too tight coupling between code.

You can use folders to decouple code by making code private within that folder. Only publicize parts of the code with your languages version of exporting.

Or you can make it private behind an entire new repo and behind an entire service. Only publicize parts of the code through api interfaces like http and make everything 10x harder for the sake of decoupling code.