They might have something to gain, but they're probably deeply invested in the tech already.
People are very hard to dislodge from such positions, unless the new positions have overwhelming benefits such as: a new environment does not support the old tools/programming language; a radical paradigm shift has happened that risks to make completely obsolete their previous knowledge, etc.
Rust is more of an incremental improvement over C++, than a radical leap forward. So C++ magicians are less likely to want to switch over.
I still use 'C'. I do this primarily because the legacy code base is staggeringly large. I'd jump on a gig doing Rust in a heartbeat, all other factors to the good.
People are very hard to dislodge from such positions, unless the new positions have overwhelming benefits such as: a new environment does not support the old tools/programming language; a radical paradigm shift has happened that risks to make completely obsolete their previous knowledge, etc.
Rust is more of an incremental improvement over C++, than a radical leap forward. So C++ magicians are less likely to want to switch over.