The reason I life profiles is they are not all or nothing. I can put them in new code only, or maybe a single file that I'm willing to take the time to refactor. Or at least so I hope, it remains to be seen if that is how they work out. I've been trying to figure out how to make rust fit in, but std::vector<SomeVirtualInterface> is a real pain to wrap into rust and so far I haven't managed to get anything done there.

The $1 billion is realistic - this project was a rewrite of a previous product that became unmaintainable and inflation adjusted the cost was $1 billion. You can maybe adjust that down a little if we are more productive, but not much. You can adjust it down a lot if you can come up with a way to keep our existing C++ and just extend new features and fix the old code only where it really is a problem. The code we have written in C++98 (because that was all we had in 2010) still compiles with the latest C++23 compiler and since there are no know bugs it isn't worth updating that code to the latest standards even though it would be a lot easier to maintain (which we never do) if we did.

It's also expected that you'll be able to do this with Safe C++. Of course the interop with older C++ code will then still involve unsafety. But incremental improvement should be possible.