I'm really sorry to hear this, but I guess it isn't unusual after that long at one job. I do wonder, though, if Mark Harmon will stay or go after his current contract is up given this development. And David McCallum is getting pretty old.
I have been watching this series since the first episode and still enjoy it. I do have to say that the years with Tony and Ziva, especially the first few years Ziva was on, were the ones I liked the best. After she left it wasn't too long after that you could tell Michael Weatherly really didn't have his heart into it anymore. I didn't particularly care for the way they replaced him with three characters. Too many. I didn't like Jennifer Esposito's character, I thought she was too bland, too much like Bishop. And I don't see why "the British guy" is there either. Now, Wilmer Valderrama is a different story. I love his character! Nick Torres has enough personality to carry off being a replacement for Tony. And so far I like what they're doing with this season's storyline. It's different and interesting. A new Gibbs. Now, with Abby leaving, I could see the new Gibbs deciding on a lifestyle change after the following season.
I would miss the show if that happens, but it has been going for a really long time. Not many series can keep my interest for that long, and with so many character changes. But who knows? They might find somebody interesting to replace Abby. I didn't think I would continue to watch after Tony left either.