For code modularity to serve the same purpose, I think there needs to be explicit language-level support, because on a medium-sized or larger project, when the

For code modularity to serve the same purpose, I think there needs to be explicit language-level support, because on a medium-sized or larger project, when the modularity exists only in the minds of developers, it might as well not exist at all. You need virtually all of your developers to understand the design and respect the seriousness of violating it, and beyond a certain project size, that isn't possible. Developers do a much better job with microservices. I think it's easy for them to respect the seriousness of designing and changing the API of a microservice. In contrast, they often don't respect or even realize the seriousness of making a change that affects the modular design of code. Language-level support for defining and enforcing module interfaces might help developers invest the same level of care for module boundaries in a modular monolith as they do in a microservices architecture, but I've yet to work in a language that achieves this.