At the end of the day, this is a business.
NCIS will continue as long as Mark Harmon is both interested and reasonable in his $ demands. Remember Gunsmoke and Bonanza continued for nearly 25 years each, despite having only one standard plot they just variated every week. Dead sailor in Norfolk, Abby or Ducky spew technobabble, Gibbs grills some dumba** who does not understand what "fifth amendment" means, then slaps the probie on the head and yells "work harder" (gee, so that is how you solve crimes, work harder, who knew) and then there is a shootout where no one actually gets shot. Fin.
As to Dinozzo, it is a business. The writers can either kill him off or have some version of him going elsewhere.
Either they simply promote him (which would have happened in the real world a decade ago) or have him go off on some secret spy mission, which sets him up for eventual guest slots throughout the JAGverse. Which is what will happen. He goes off on some quest, probably with Ziva (who will not be seen) deep undercover. He bubbles up on LA or NO in a couple years the save the day.
As to the minor characters, nothing is going to happen to anyone.