It cannot crash Python; it can only crash hypothetical third-party programs or libraries with deficient error checking and unreasonable assumptions about input data.
The error checking isn't necessarily deficient. For example, a safe and legitimate thing to do is for third party libraries to throw a C++ exception, raise a Python exception, or delete the half surrogate. Any of those would break one of the use cases people have been talking about, namely being able to present the output from os.listdir() to the user, say in a file selector, and then access that file. (and, of course, you haven't even proven those programs or libraries exist)
PEP 383 is a proposal that suggests changing Python such that malformed unicode strings become a required part of Python and such that Pyhon writes illegal UTF-8 encodings to UTF-8 encoded file systems. Those are big changes, and it's legitimate to ask that PEP 383 address the implications of that choice before it's made. Tom