The risk might be small -- but is it a risk they need to take at all? We're talking about a very mature project here that more or less needs to move at the speed of standards bodies, which is basically glacial. Being the first to implement C++ 29 probably isn't that big of a deal. People do not upgrade their compilers daily, shit, most projects stay on the same version for years, so optimizing for velocity seems like a bad idea here.