I think we're genuinely getting to the point now where the majority of "LANG=C" cases are misconfigurations rather than intended behaviour. We're also to the point where:

- on Mac OS X, binary system interfaces have been handled as UTF-8 by default since 3.0
- on Windows, as of 3.6, the OS native binary system interfaces are now bypassed entirely in favour of transcoding from UTF-8 to UTF-16-LE 

So I think for Python 3.7 it makes sense to do the following on other *nix systems:

- very early in CPython startup (even before argument processing), if the detected locale is "C", force it to "C.UTF-8" if possible, and print a warning either way
- add a PYTHONKEEPASCIILOCALE environment variable to turn that behaviour off

I do think we actually want to *change* the C level locale in the process though, as otherwise we can expect to see weird interactions where CPython and extension modules disagree about the default text encoding.