This is probably the reason why our opinions vary. I believe rewriting existing libraries to be safe would be a tremendous amount of work. This estimation comes from my experiences with Rust. Safety guarantee hugely affects the interfaces of the libraries. You cannot "just" gradually increase safety of your library. It is basically a redesign. At least with what Rust currently provides this was my experience. And the method suggested by Herb Sutter seems to be very similar to Rust's. So I expect the same churn to happen in the C++ ecosystem, and it would effectively split the ecosystem into two: safe ones and unsafe ones.
Let me rephrase myself again: if adding safety to C++ in a gradual manner was that easy, Rust would have not been invented in the first place. The Mozilla guys could just use C++. There's a reason why Rust had to exist, despite all the "reinventing the wheel".