That's a good point, Teresa.
One thing that has got me to the end of writing three books** (publication is an entirely different matter), is wanting to know how each turns out. Even where I have a really good idea where each plot strand is headed, this is not the same as having the fully fleshed-out story strands' completions on the page in front of you. (Of course it isn't; otherwise we'd be suckers for reading book synopses rather than the books.)
** - I've always been a better starter than finisher, so when I completed my first draft of a novel, no-one could have been more surprised than me.
One thing that has got me to the end of writing three books** (publication is an entirely different matter), is wanting to know how each turns out. Even where I have a really good idea where each plot strand is headed, this is not the same as having the fully fleshed-out story strands' completions on the page in front of you. (Of course it isn't; otherwise we'd be suckers for reading book synopses rather than the books.)
** - I've always been a better starter than finisher, so when I completed my first draft of a novel, no-one could have been more surprised than me.