I think that you should try and not ruminate on this too much. It's out of your control whether or not your therapist leaves, and so I wouldn't waste too much energy thinking about it. Try talking to your t about it and let him/her know that you would like as much notice as possible if they do end up leaving, that way you have time to wrap things up and have a good termination and transition to a new t.

I do think thought that your t is not going to leave you hanging, He/she will most likely recommend someone else for you to see, and help you wrap things up in a good way. They are trained to help end relationships and so hopefully all will go smoothly.

Again these are all things to talk to your t about. I wouldn't freak out until you know that he/she is really leaving.