Why isn't Jeremy Carver writing the finale? It's pretty simple, his project Frequency, a pilot that he pitched last year at NBC and was passed over, has been picked up this year by The CW. It's a remake of the film by the same name from 2000. He is Executive Producer of that show and writing the pilot, so he's tied up with that right now. The pilot is in casting stage and close to shooting.
I've heard the question, does this mean Jeremy Carver is leaving Supernatural? No, but his role could change. It all depends if The CW picks up Frequency. As of right now, they've only committed to the pilot. The decision on pickup won't happen until May. The CW has ordered six pilots and will likely pick up two to three for series. They don't have a lot of slots. So Frequency is really going to have to impress to get a shot, because one of those pilots is a high profile Kevin Williamson/David Nutter project that seems like a sure thing.
What if Frequency gets a series order? Jeremy Carver becomes a showrunner for two shows. Chances are the new show will take most of his attention at first. He likely won't be writing episodes for Supernatural anymore (or will do a rare one here and there). That's what's nice about a veteran show like Supernatural, it practically runs itself. Phil Sgriccia this year took over a lot of the production management when Robert Singer stepped down. Andrew Dabb has taken over the lead role in the writers room. The same structure would stay in place.
For those wondering how Carver can do both, this model is already taking place at The CW. Kevin Williamson and Julie Plec, Executive Producers of The Vampire Diaries, have gone onto other projects while keeping their titles on The Vampire Diaries. Caroline Dries has been the lieutenant in charge for that show while Michael Narducci has been handling The Originals. Plec even has a third CW show premiering soon, Containment. Greg Berlanti, Andrew Kreisberg, and Marc Guggenheim are all dividing their time between Arrow, The Flash, and Legends of Tomorrow. Berlanti also does Supergirl and has more pilots going this year, yet he's still tied to his original show, Arrow.
So, it's all good. I say good luck to Frequency in the pilot process and good luck to Andrew Dabb for the finale. It's a well deserved honor for him. Now we just need that official announcement for season 12, which no doubt should be coming soon (in March anyway).