Actually, there's also a case for sexism
in this situation.
By switching Seven for Harry, the balance of power tilts dangerously
towards the female in the star trek tv equation.
By simply adding Seven to the cast of Nine, it would have diluted the storylines devoted to "other" characters.
I liked Kes... but I came to love Seven. I was sorry to see Kes go, but can't find it in my heart to regret Seven's arrival EVEN though it diluted the presence of my other fav female... B'Elanna Torres.