Answered this question, and brought the discussion back here.
You can not mix C++ directly into the Swift files, but I do not think there are many languages where that is allowed. The ability to add C++ to ObjC files, is a quite unique feature. I have never felt the need to mix these things though.
Currently there is no native cross platform version of cocos2d, based on native Apple technology. While C++ will work in Xcode, it still is a kind of hybrid.
With the power of the cocos2d idea, it is not a far fetch to imagine a Swift version. Go, Rust and other languages might be great, but I want to build something for the Apple toolchain. I think this is the best development environment there is, and to make great games, you will always need great tools. Swift will not go away anytime soon.
You are correct though, that utility classes will not work anymore. However, I will bring back native openGL support, meaning that anything cool written for anything supporting openGL, will work with an absolute minimum of changes. In general, I will make sure that PureSwift gets easy to understand and expand on, so that utilities will be easy to implement.