I am genuinely curious to hear a good reason why timestamps should be preserved by distutils. I cannot really imagine what could possibly break. As I said, this is the standard for many non-Python projects and it seems to work just fine.

I can see one potential backwards-compatibility issue for projects abusing distutils to do things unrelated to installing Python packages. So I understand that you don't want to change the default in the copy_file() function, but only in the methods calling that function.