Then again it seems some of the most discussed problems with story planning. And ways to avoid the problems. I wonder how many unfinished stories never even enter that kind of a discussion.
One thing that I've observed, is that many 100+ chapter serials are written by new authors, who are trying to recapture the stories they've been following for a long time. But the burn-out rate among those authors is high, and many never write anything else, not even counting those that just drift off and die, never being completed.
Generally (and I'm sure DS will jump all over me for this assertion), most teachers, writers and other sources all recommend starting small.
I kind of took a middle ground. I'd long been a fan of the extended series, but my first story was chopped up into more reasonable pieces. It ended up as 6 books and over 1,000,000+ million words. Still, breaking it up allowed me to strengthen each book individually, rather than producing a single, weaker continuous story.
I thing I would like to see, and haven't, is for those authors of those epic serials, once they finish, to go back and edit them, cutting out the fat and the unproductive, clean it up, add foreshadowing elements and produce a more focused, professional finished work. It would be interesting to see how much the overall story would improve in that case.
Note: In the case of my first story, I also ran into a number of plot holes, and had to rewrite the entire story a number of times. Each time I did, I ended up coming up with a MUCH stronger and coherent story, rather than simply continuing on from where I was.