I think we can agree to differ on this one, but I do agree with a lot of what you say. Particularly irritating was that RTD teased this possibility by turning Donna into someone who could genuinely be an equal to the Doctor, then tearing it away. River Song could also have taken this role, but got ... erm, emasculated. The advantage of a female doctor is that the writers pretty much can't sideline the main character in that way.