I think knowing how to write modular code is somewhat separate from the advanced "library writer" features in C++. Anyone can write clean and modular code without them, just as you could before they existed. You don't lose expressiveness per se but from a library user perspective it requires a great many more things to be explicit, writing a lot more boilerplate, and a larger API surface area that you have to learn. The new C++ features give the library writer the option of investing in more elegant, flexible, and concise expressions of modular code with little additional work but requiring stronger C++ skills to elevate the modularity in this way.

It is an ease-of-use improvement as much as a modularity improvement and provides some evolutionary pressure to design better interfaces to modular code.