I think you are right to position atomics and variadic templates as among the most consequential features of C++ in the last couple decades. It made an enormous number of things possible.

One thing I give the C++ feature gods credit for is recognizing material deficiencies in systems languages and unapologetically trying to fill those gaps starting with C++11. The addition of atomics, strict variadics, constexpr, lambdas, concepts, lifetime annotations, etc has been transformative for C++.