Others have already pointed to the distinction between code duplication and runtime duplication. Regarding the development experience: I think it's a tug and

Others have already pointed to the distinction between code duplication and runtime duplication. Regarding the development experience: I think it's a tug and back-and-forth kind of dynamic. Modularity has exactly the kind of effect you talk about, until it doesn't. At some point, the cost of communication (between people!) responsible for different modules can increase until it's a net negative to try to adapt a module, or even find out it exists and is suitable. Of course, a monolithic project of comparable size has likely collapsed from its weight into a ball of mud. Development tools have come quite a ways, but the science of getting teams to develop software together, perhaps not so much.