Well of course Seven of Nine was a fantastic addition to the show, but getting rid of Kes wasn't a good idea. All Brannon says is that Seven was a great addition, and that season 4/5/6 were some of the best trek stories.
I agree with him that the concepts of many seasons in 4/5/6 were some of the best in Trek, and that Seven of Nine was a great addition to the show.
On the flipside I would say that looking at the bigger picture, those seasons were disjointed which negatively affected the show as a whole. And Seven of Nine didn't need to replace another character that had a lot of potential.