Well, that's why we're here. As fanfiction writers, readers, creators, we keep the story going when Scott cannot.
Plus, there is the slim chance that the fourth book won't be as good as the other three. He could potentially shoot himself in the foot with that. ¯\_(ツ)_/¯ he is doing this for a living
I just know that when I first read Leviathan, I couldn't get enough of it. My tiny little mind absorbed the characters, and I too wanted there to be more, more for me to absorb.
Now that I have - sort of - left the fandom behind me, my hunger for more gently slipped into a state of contempt. I am glad he didn't make another, and I glad that just the other day - I could recommend the series to someone because I knew it would be good.
But yes, I agree, another book would be dope, but quite unnecessary.