In existing code bases, new subsystems can be fully modern. Old code can be modernized incrementally, e.g. replacing uses of naked pointers with smart pointers

In existing code bases, new subsystems can be fully modern. Old code can be modernized incrementally, e.g. replacing uses of naked pointers with smart pointers and then deleting destructor code, or replacing C-style for-loops with modern range-style for-loops or calls to standard algorithms, passing a lambda. Many class declarations can be eliminated by using lambdas inline. A sweep throughout a big codebase just modernizing one kind of thing may take only a few days. This typically results in deleting a great deal of code, and all the bugs in the deleted code evaporate. Deleting masses of code always feels great.