Yeah, tools can add support for them, but they're fundamentally a whole-program-optimization in a build chain that's 99.9% file-by-file.

Supporting const enums will, by necessity, greatly reduce the maximum performance that toolchain can achieve, because you have to evaluate the entire project to tell whether `Foo.BAR` should be left alone or replaced with a constant defined elsewhere. And in the worst case of an ambient declaration, "elsewhere" could be any file in the project.