Not the writers. Berman and Farrell unable to come to an agreement. I don't see anything wrong with bringing in Dax in a new host. Since they already told how trills work, it's the logical way to handle Jadzia being off the show. This is the opportunity to show as well as tell.
And it's not an insult. People fail to find a way to work together all the time. Too bad, but it happens.