NCIS has had several main character exits since its start in 2003, so one would think it’s at least conditioned to survive a scenario where Harmon announced his exit.
With that said, it’s possible NCIS would continue on without Gibbs, much like it did without actress Pauley Perrette in Season 16.
Perrette’s Abby Sciuto was a main character on the program since the show’s beginning, much like Harmon’s Gibbs, and the series remains as popular as ever in her absence.