At the time it aired in the late 90s, I would have said let Jadzia be recurring and/or bring her back for the finale. In hindsight, I am not so sure. Ezri has grown on me over the years. From the producers point of view, if there was any character that would be easy to kill and replace, it's Dax. You get someone new who's not really new and still has connections to the cast.
I must say, I always thought it was ironic that (on of the reasons) Terry Farrell left because her character didn't get enough storylines, but as a result of her departure the Ezri Dax character got several.