The problem microservices try (tried?) to solve isn't about namespaces, it is about too tight coupling between code. Whether that tightly coupled code sits in a subdirectory or in a different repo doesn't matter.

It can be benefitial to maintain well defined interfaces at boundaries between certain parts of your code. It can also produce a lot of work and add complexity. But beyond a certain scale adding systemic boundaries and honoring them isn't something you should avoid.

Devs who do microservices just tend to go too far too early.