I'm sensitive to the argument that public/private rules improve maintainability of libraries by shrinking the exported API. Maybe, but that's a maintainability

I'm sensitive to the argument that public/private rules improve maintainability of libraries by shrinking the exported API. Maybe, but that's a maintainability argument, not a usability/reusability argument. It's hard to predict which part of your program developers are going to need. Also, 'private by default' prevents the gradual modularization of code that happens in languages lacking visibility rules.