It would definitely making interop between C# and C++ much easier. It would make interop between C++ and any language much easier. The only way to do it today is to extern "C" everything. It's ugly.
This doesn't work with C either or does it? C has a reasonable ABI so maybe we should get to the point where exchanging C++ libraries is as easy as doing this with C
The point is there is no standard C++ (or even C) ABI because it is not possible to standardize one. At best you can standardize a platform specific ABI. Which is in fact what happens in practice on most platforms, but it is up to the platform mantainers and the C++ standard itself can't have nothing to say.
Regarding the C ABIs, they are the lingua franca for interchange between languages because, a) as C is semantically poor, it is the minimum common denominator, and b) it is often the OS ABI.