If C++ didn't exist, are you saying that people would not be capable of learning C in that case because they have to deal with null-terminators? IMO C is actually an easier language than C++ because there is more to the C++ language over C, even though C doesn't have a lot of the features that C++ has by default, and in which case typically means that you have to write your own implementation rather than using some generic implementation that already exists for something.
You've got to start somewhere is all I'm saying. But to start off with std::string, then learning about the allocators and template stuff involved, along with polymorphism, and iterators behind the mystery of the std::string typedef, to me, is not much easier than just learning how null-terminated strings work.
I can see your point, and I also agree with it, as well as my own sadly... For that reason, I can see this is a very subjective matter and that there's no real way to dictate the way someone should learn about strings because everybody has a different way of learning, and there are many other factors that decide how well someone will learn a concept one way vs another, including previous programming history, and others.