It's reaching its organic ending - The series has been on air for nearly 20 years, and it's running on narrative fumes.

Mark Harmon is aging - Since he's nearly 70, and television shows are grueling to produce, when he decides to leave the show will be effectively canceled.

Ratings dropping - While the series is still pulling in impressive numbers, its ratings have definitely declined. Continuing to do that will make the decision to wrap easier.

The cast changing - Most of the longterm cast members, excluding Harmon and David McCallum have left the series. The new cast members are carrying the load; however things have definitely changed. That effects audience appeal.

The unknown - The cast lead is older, as is one of the primary characters. The show was roiled several years ago when a major cast member quit under a cloud. After nearly 20 years on the air, there are too many variables to ensure that the series will continue uninterrupted.

All things must come to an end. The same should be said to Law and Order: SVU and especially The Simpsons.

i dont even watch it anymore it need to end soon before it on the air for to long

Yes, I think NCIS will finally bow out after being on for so long. CBS should end that show at the end of 2019-2020 TV season once if Mark Harmon leaves.

I stopped watching after Tony left. Sometimes shows just last too long.

One other factor must also be considered: The Big Bang Theory being gone from new episodes.

NCIS and TBBT were the top shows on CBS during this decade, and now, with the latter series ending last May, it's not going to feel the same after the 2019-20 season. Also, I quit watching NCIS live some time ago. I could foresee at least an 18th season of the original NCIS series before all is said and done.

NCIS is “The Mark Harmon Show”. It will end when he says he’s ready to retire.