A very mature language, rapidly evolving, with an enormous volume of well proven libraries and tools, or a cutting edge, barely production level (with a breaking changes language), tiny library new toolset. And you question why someone doesn't just transition? Is that serious?
Oh no you didn't.
There's an argument to be made in Rust vs C++ but to my knowledge there hasn't been a breaking change in Rust since 1.0 and there's been a ton of work to keep it that way.
The key is, as you said, you aren't even sure there were ones. That's how we like to keep it, and that's part of where all that work goes. The rest goes into making sure nothing breaks accidentally :)
Trivial example: shortly after 1.15 was released, a new API was found to have a soundness hole, so a breaking change was made to fix it in 1.15.1.
I love Rust, and I've been trying to get a hobby project written in it - but it really stinks that I can't just rely on my distribution to keep dependencies up-to-date like I can something written in C or C++.