I realize that the following would not move the ISAF one bit, but...
IMO, by excluding sailors on grounds other than ability (e.g. those who have sailed in a non-ISAF "world championship"), just on principle the ISAF forfeits the legitimacy of any claim that an event they sanction is in fact a real world championship. It no longer provides a defensible determination of the best willing and able competitor in that class.
So much for protecting the integrity of the sport.
Mark.