But a borrow checker does not change the set of possible programs. It only identifies the subset of possible programs that would need "unsafe" blocks.

In the absence of a borrow checker, libraries are left to carry the responsibility to define an API that is hard to misuse. Such an API could be slower than what one would define for a library to be used under a borrow checker. But code written to satisfy a borrow checker is sometimes slower than code that doesn't need to.

Once you have Turing completeness you have the full set of possible programs, end of story. Languages aren't about changing the set of possible programs, they're about making good ones easier to write, and bad ones harder.

Yet, C++, D, and Rust are much more like one another than any of them is like any other language. Your Turing Completeness tells you less than nothing about any important distinction between them.