In the game of chess sometimes you know you lost when your opponent is one move away from the check mate. He has his pieces in place and you know it is over. It has nothing to do with giving up but just being realistic (or just not that great of the chess player). Most of the time you just move a piece in futile attempt to prolong the game but once in a lifetime you see the light and put up a fight and win.
Now last book put all the pieces (Dirk and the crew) in a place where check mate can be executed toward the entire series. I mean Dirk is in charge of NUMA and married, Al is by his side, Admiral is in the White House, kids are incompetent of surviving on their own without help from the dream team (who is now retired).
It can be over right now. Or Dr. Cussler can also write another book in futile attempt to save the series where he puts Dirk's kids on a mission and brings "dream team' out of the retirement to save them. That scenario would give him one more book (kind of corny in my opinion). Or we can talk about that one in the lifetime move that saves you from check mate and wins the game. But I guess Dr. Cussler is the one who has to make that move now.
Matt
I have mentioned this in a previous post that there should be adventures of Dirk and Al that have occurred between the earlier published books when they were younger. This would keep the original two going while the NUMA and Oregon Files can continue. And Summer and Dirk Jr. can have their own series of books or join the NUMA Files series.