I don't agree. Any input string that violates the strcpy invariants (must be a valid null-terminated string with length <= destination buffer size) will also violate the strncpy invariants if you subsequently treat the destination buffer as a string.

Since treating the destination buffer as a string is very tempting to do given that it came from a function named "str*cpy()", you've basically added another "Gotcha" to strcpy() without gaining any safety. You're basically trading one form of undefined behavior that's explicit in strcpy for another form of undefined behavior when a developer mishandles the return value of strncpy.