I seem to remember Sakurai saying that this would be the last one. I guess he either doesn’t have much of a say or that it made too much money to pass up on another….
Maybe my memory is faulty but I seem to recall every single sequel was going to be the last one. Them Nintendo saw the profits and cuts him a bonus so he turns around and says one more.
Sakurai's independent. Nintendo always goes to him first when it's time to make a new entry, but Sakurai's under no obligation to accept. Nevertheless, he keeps coming back to the table, so it's probably fair to assume that he enjoys directing this series.