>Devs will monkey patch and reach into other module's internals without a second thought. I can't imagine the monstrosity that would exist if the network boundary wasn't there to stop them ...

We have a tool in our modular monolith, which is like a linter which checks architectural violations like that. If someone tries to bypass the API layer of a module and peek directly into the internals, it refuses to build (and CI/CD refuses to deploy/release). No need to replace a basic linter with a network boundary...