If I turn on C++23 and get a handful of errors over those million lines of code I will spend the week or two needed to rewrite just those areas of code. That is much easier than rewriting everything from scratch in rust. Even if we just wrap all needed C++ APIs in C so we can use rust that is a lot of effort before all our frameworks have the needed interfaces (this is however my current plan - it will just be a few years before I have enough wrappers in place that someone can think about Rust!)

Note too that we are reasonably good (some of us are experts) at C++ and not Rust. Like any other human when we first do Rust we will make a mess because we are trying to do things like C++ - I expect to see too much unsafe just to shut up Rust about things that work in C++ instead of figuring out how to do it in safe rust. (there will also be places where unsafe is really needed) I want to start slow with Rust so that as we learn what works we don't have too much awful code.