It could but Rust already solves the issues this is aiming to solve but with fewer steps. I don't know what Mozilla's policy is on adding Rust vs C++ for new features.
Safe rust solves the memory safety problem. However, there is still the possibility of a logic bug causing rust to touch something it shouldn't.
The sandbox approach is about adding a second level for malicious code to bypass. You now not only need to find a way to get past the code, you also need to find a way out of the sandbox.
It's a little like running your apps on a server with highly restricted permissions. You do that so the app compromise limits what is exposed.
And conversely, if a C/C++ component is put into a wasm module, work that's needed to clearly specify the API can be re-used for any possible Rust rewrite in the future.
