The ending of Franklin's Autobiography is kind of a problem because, well, there isn't a proper one. Franklin was in the middle of writing Part 4 when he died, and at that point he's only just finished telling us about the French and Indian War. He hasn't even gotten to the Revolutionary War, which was, as you know, pretty significant. Plus, Franklin kind of had a big part in that.
So this causes us some problems when we're reading. First off, this is the kind of issue we'd have reading any autobiography. Since an autobiography is a life story written by the person who lived it, the end of the story is never going match up with the end of the person's life. It's just not possible. Yet we might want to think about how anyone can tell his or her life story as a whole without mentioning a key event in that life – leaving it. The story's never going to be complete in that sense – it's a problem with the genre. Usually, though, an author can lead up to or close with a significant event, like success or marriage or something, that helps his or her narrative feel like it's "finished." Franklin didn't get the chance to do that, though – he just stopped writing. (For an interesting comparison, see our guide to The Diary of Anne Frank.) In that sense, his death does become a part of the book's ending: you just have to read between the lines. It's the cause, not the effect.
On its own, the description of a minor historical event – asking for fairer taxes – doesn't really pack a lot of punch as the ending to a story about the life of someone as significant as Franklin was; it's probably fair to say that if Franklin had stuck around a few more years, he would've written some more stuff. Maybe he would even have come up with an ending that acted more like the "traditional" kind of ending we expect from autobiographies and life writing. Think about it – Franklin goes to lots of trouble in the other sections of his book to tell us why he starts writing about that time or place, and what motivated him to do so. His voice as "author" vs. "subject" is way more present. He doesn't get a chance to do that here, and we're left to wonder what else he could've said.