@awnlee jawkingChiming in (more or less on cue):
It's certainly true that the further into the future the story goes, the less well it's mapped out.
That said, for essentially the entirety of Book 1, nothing beyond the next 10-20 chapters was really mapped out. Book 1 is pretty much all seat-of-the-pants writing. There are a few exceptions (for instance, parts of the summer vacation I generally knew were coming).
By the time Book 1 finished, parts of Book 2 were settled, parts were in flux. Some of the bigger plot threads popped up in the middle.
Much more of Book 3 was plotted out when I started it than was the case with Book 2, though I still have some big swaths of time where... something happens. What? Not sure.
Long-term, there are some major events mapped out for Books 4, 5, and 6 (or, really, Senior Year, Freshman Year college, Sophomore year college). Another major event should happen around 1987 (it'd be a spoiler now - however, once you see where one storyline is going in Book 3, 1987 becomes obvious).
In a way this may make it worse :) since I'm being pretty clear that it won't be 'done' at least anytime soon. There's just too much story to tell, and by the nature of some of the characters and plotlines some things can't be resolved by the end of high school, for instance.
My writing method is moving more and more towards scoping out the current 'medium-sized picture' and seeing what happens in the smaller moments there. If something happens that screws up something bigger, either it's a mistake or (more likely, really), I screwed up the bigger picture. None of the previously mentioned major events are really in that category, though - I can't see them derailing.
I'm trying to pare down (somewhat) the stretches where things are 'quiet'. If I can skim it, I'll skim it. Of course, that's in my judgement :) A number of readers would've been fine with 90% of Summer Vacation 1981 being skimmed.
There's a lot of introspection, definitely, and it won't go away. Getting a second chance at life lends itself to that. There are plenty of interesting 'Why is this different? Am I doing the right thing? What do I owe people who I could help?' type questions.
Replying to the big question: I have no intention of leaving it unfinished. In fact, I was pondering today how to write a broad, as-satisfying-as-possible (probably at best 'moderately satisfying') wrap-up in case some disaster overtakes me and how to make sure it gets posted.
I didn't start posting Book 1 before I'd written the entire first draft, but that's not true of 2 or 3. Obviously it could be unfinished.
On the other hand, the same is true for A True History, A Well-Lived Life, and many other long-form SOL stories, and I've enjoyed reading those. Best intentions of their authors aside, life happens, and sometimes it abruptly changes plans.