I mean the argument is made that Rust is not mainstream, but if you were to pull in e.g. Java or Javascript apps in that comparison I'm confident it would make C++ look like a non-mainstream language too.
edit: numbers say I'm wrong, C++ is more mainstream than C#, C is more mainstream than Java: https://www.tiobe.com/tiobe-index/. I mean I doubt this source (it seems to indicate objective-C, which is all but superseded by swift, is on the rise) but it's at least one datapoint.
edit: numbers say I'm wrong, C++ is more mainstream than C#, C is more mainstream than Java: https://www.tiobe.com/tiobe-index/. I mean I doubt this source (it seems to indicate objective-C, which is all but superseded by swift, is on the rise) but it's at least one datapoint.