It would have been hard to recover from losing Brooks, especially after season 6. If Brooks left, Jake would probably leave too (back to Oregon to live with his aunt, or to New Orleans) as he wasn't really quite ready to live on his own yet. If Jake left, Nog wouldn't have nearly as much of interest to do. Farrell was already leaving. Kira would be very distressed about the Emissary leaving. If they brought Ezri in, she wouldn't have that strong friendship ready made with the new CO. The relationship between Bajor and the Federation would be seriously hurt. Altogether it would have been a major blow to the show.