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Yes, that's the point. *Every* case I've seen where the locale encoding has been reported as ASCII on a modern Linux system has been because the environment has been configured to use the C locale, and that locale has a silly, antiquated, encoding setting.
This is particularly problematic when people remotely access a system with ssh and get given the C locale instead of something sensible, and then can't properly read the filesystem on that server.
The idea of using UTF-8 instead in that case is to *change* (and hopefully reduce) the number of cases where things go wrong.
- if no non-ASCII data is encountered, the choice of ASCII vs UTF-8 doesn't matter
- if it's a modern Linux distro, then the real filesystem encoding is UTF-8, and the setting it provides for LANG=C is just plain *wrong*
- there may be other cases where ASCII actually *is* the filesystem encoding (in which case they're going to have trouble anyway), or the real filesystem encoding is something other than UTF-8
We're already approximating things on Linux by assuming every filesystem is using the *same* encoding, when that's not necessarily the case. Glib applications also assume UTF-8, regardless of the locale (http://unix.stackexchange.com/questions/2089/what-charset-encoding-is-used-for-filenames-and-paths-on-linux).
At the moment, setting "LANG=C" on a Linux system *fundamentally breaks Python 3*, and that's not OK.