I think we've all read enough fiction to know [article 37], in some shape or form had to happen, but the structural scaffolding of the story is so well layered with characters we care about that we still gasp, maybe shed a tear when it hits.
Ya know... when someone says "such and such had to happen", because of some theory of over-all story structure a la Joseph Campbell - I have to wonder why we don't just program the computers to automatically crank out stories to fit the formula.
I get this a lot. I think the idea has been transmogrified. It's not that it has to happen because of Joseph Campbell. It's because it has to happen in support of the story. Campbell merely pointed out the pattern that has been in place in human's story telling as far back as he can find stories.
The Incident in episode didn't happen because Joseph told me I needed to make that happen. The story required the traumatic, shocking event to spark the necessary change in the character - to motivate the action to follow. How that's applied, where it happens, what it is...all that is up to the story teller. But when all is said and done, there's pretty much always one of these catalysts in every story because, without them, it just becomes an essay.
There are plenty of ways to end the series. Any of them that maintain the status quo, fail to provide sufficient motivation to grow, or otherwise try to dodge the reality that I believe the story requires would be the end of Ishmael Wang for me. I would have kept my promise not to kill him, but there probably would be no other stories with him from me. Not that there couldn't be more stories. They're just not sufficiently interesting for me to invest in telling them.