I'm assuming the context of the question is "People say they see it replacing C++ all the time". People actively want a C++ replacement, and a lot of the featu

I'm assuming the context of the question is "People say they see it replacing C++ all the time". People actively want a C++ replacement, and a lot of the features of Rust specifically target the pain points of C++. The pain points are painful enough, and the differences in Rust are soothing enough, that people are willing to invest in Rust as a future replacement. People don't actively want a Ruby or Python replacement; they're good enough for now. There isn't much in Rust that a Ruby or Python programmer will look at and say "that problem has caused me issues for years". It doesn't offer enough over what they already have to invest in such a young language. If Rust succeeds, I wouldn't be surprised to see it at least sitting alongside Ruby and Python, but by that point the ecosystem will have built up enough around it that you're not losing so much by switching.