Prince of Spires
Istar
I agree with the first part, but I'm not sure the second part is necessarily true. There are a lot of reasons people don't finish. I think the two main ones are finding that writing a book is a lot of work and running out of steam.I'd be prepared to bet there are more non-finished books than finished across the history of literature (and attempted literature). And the reason? They ran out of interest or ran out of ideas...
Running out of steam is related to running out of ideas. The difference is that for me running out of ideas seems very definitive, while running out of steam (because you've written what was in your head and now you have no idea where to go next) can be fixed if you're willing to sit down and think it through. In the end, ideas are cheap and a dime a dozen. The execution is a lot more important then the idea. And while the idea might not be novel-sized, that doesn't mean it's not novella-sized. But people don't continue with their idea to write that novella, they just stop writing.
Of course, some ideas are just bad. Not every idea is worth finishing. But if you've started several novels (like the OP), then I believe that at least one of those will be worthy of a book, even if it's only 40k words.