I think "you should never do a full rewrite" is something of a reactionary response to the juvenile idea new devs and interns get every now and then. But sometimes, a rewrite really is a good idea.
But in this case, per the announcement ( https://blog.torproject.org/announcing-arti/ ), the reasons are pretty convincing. The major reasons are, unsurprisingly, all around memory safety.
I've tried compiling large projects in Rust in a VM (8GB) and I've run out of memory whereas I am sure a C/C++ large project of a similar size wouldn't run out of memory. A lot of this tooling I had to compile myself because it wasn't available for my Linux distro (Debian 12 at the time).
A lot of the tooling reminds me of NPM, and after spending a huge amount of my time fighting with NPM, I actually prefer the way C/C++/CMake handles stuff.
I also don't like the language. I do personal stuff in C++ and I found Rust really irritating when learning the language (the return rules are weird) and just gave up with it.
I guess you and me live different lives because I have spent far more time messing with ancient C/C++/CMake/Automake/Autoconf/configure/just have this particular .so file in this particular spot/copy some guys entire machine because this project only builds there/learn and entirely different language just to write build files (CMake sucks and 5 other alternatives are just lipstick on a pig) etc. etc.
I am of the opinion that half of Rusts success is based on that fact that C/C++'s tooling is annoying and ancient. People want to write code not mess with build envs.
I am a hobbyist C/C++ developer and have a intermediate size code base that builds on Linux, Windows (MinGW and MSVC) and MacOS and I didn't do anything particularly special. What I did do is setup CI early on in my project. So I had to fix any issues as I went along.