A rewrite of your C++0x codebase that's grown from 2009 until now will most definitely fix loads of memory bugs, since C++ has very much evolved in this area since then. The added value of the borrow checker compared with modern C++ is a lot less than compared with legacy C++.

That said, I still think it's a rather weak argument, even if we do accept that the rewrite will do most of the bug removal, since we aren't stupid and move to smart pointers, more stl usage and for each loops. "Most" is not "all".