My problem would be that about 40,000 words in, I'd suddenly think up a cool ending that would require me to go back and change or add stuff to the beginning parts. That happens to me all the time in my pantser method of writing. My way, it's no big deal. I just make a note and fix things when I edit. But when they're already out there, there's nothing you can do. You just have to suck it up, ditch the way-cool ending, and move on. That'd kill it for me.
I'd have to write the whole thing out, then release it. And if I did that, if it's good enough to let people read, I might as well try to sell it.