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.
For me, I had a very good experience rewriting a project in Rust (from Python). It was just an improvement in every regard (the project was easier to build and distribute, it was a good opportunity to rearchitect it, and the code ended up 20k times faster.) So, I have the opposite feeling when I see titles like these.
I also feel the opposite about the tooling. For me, cmake, npm, pip, maven, etc. all live in this realm where any invocation could become a time purgatory. The big thing that makes me like Rust is that I've never had that experience with cargo. (In fact, pip was the primary motivation to move away from Python to Rust. Everything else was just nice.)
I'm not saying this to convince you to feel otherwise, I just wanted to offer how I felt about the same topics. I grew up with C and Java, but Rust clicked after I read SICP and followed along in Racket. I could probably fill a page of small grievances about Rust's syntax if I had to.
Anyone that been on a "rewrite" knows that often the end result will look like the previous implementation but in <new thing>.
So what I see is a lot of development effort to re-solve problems that have already been solved. I think Ubuntu did this with the core-utils recently (I don't keep up with the Linux dramas as there is a new one every week and tbh it isn't interesting a lot of the time). They ended up causing bunch of issues that I believe were already fixed years ago.
There are issues with things in Linux land that have been issues for years and haven't been resolved and I feel that development effort would have probably been better spent there. I don't pay canonical employees though, so I guess I don't get to decide.