Like C always will, C++ mostly still echoes the diversity of hardware architectures and runtime environments out there. That diversity is still broader than many people who don't work in weird specialties and fringes realizes, and it's useful to have a "modern" language that respects the odd quirks of those systems and how you sometimes need to leverage them to meet requirements.

If you're just writing application software for consumers or professionals, or a network service, and it's destined to run on one of the big three families of operating systems using the one of the big few established hardware architectures at that scale, there are definitely alternatives that can make your business logic and sometimes even your key algorithms simpler or clearer, or your code more resistant to certain classes of error.

If you look at Rust and see "this does everything I could imagine doing, and more simply than C++", there's nothing wrong with that, because you're probably right for yourself. But there are other projects out there that other people work on, or can expected themselves working on someday, that still befit C++ and it's nice for the language to keep maturing and modernizing for their sake, while maintaining its respect for all the underlying weirdness they have to navigate.