Stories are always written to have an ending no matter how long it takes to get there, whether it be a trilogy or a massive series. No matter how long the narrative the reader is always waiting for that ultimate climax and is reading to have closure at some point. As a comparative blog for Major Authors, I'd like to compare the endings to the two trilogies from the (in my opinion) two major authors in this class: J.R.R. Tolkien's The Lord of the Rings and Philip Pullman's His Dark Materials. Both of these trilogies seem to end on an seemingly anti-climax that should have ended a chapter or two earlier.
The last couple chapters in The Return of the King revolve around the Hobbits saying their goodbyes to their multi-racial friends and heading back to the Shire. As they return to the Shire we get an extended chapter that feels more like a short story as an afterthought to make the final book longer than it should have been. The Hobbits are forced to get the remainder of Sauraman's henchmen out of their land so that the Shire can be rebuilt back to their comfort zone. This isn't necessarily a bad chapter, but it takes away from the finality of the destruction of the Ring and the restoration of Middle-Earth. It seems as though Tolkien couldn't stand the thought of a short (compared to the previous two novels) final book and needed to tack on a light-hearted ending that forces the reader to return to the Shire one last time.
As I've stated in the discussion forum, I believe this final chapter seems to be more of a companion piece that could have been a separate short story released after the trilogy was published. It does not seem to serve a purpose other than to drag the reader through an anti-climatic ending when we have already had at least two different endings that suffice to end the trilogy on a high and optimistic note. We get closure when the Hobbits leave Rivendell and we also get it when the other main characters say farewell. The Shire saving chapter is nice and it fits with the world, but not the tone of the rest of the novel.
The Amber Spyglass could have done without a few things, namely the last ten or so pages. Mrs. Coulter and Lord Asriel have already killed the main bad guy about a hundred pages or more before this ending, which really takes away any chance of an ultimate climax, but seeing Will and Mary almost forcibly become friends and roommates, along with Lyra deciding to go to boarding school was not the most epic of endings. Perhaps the best way to end it would be the closing of the window, the destruction of the knife and Lyra sitting in the garden. No dinner scene, no Will and Mary becoming chummy, but a serene and somewhat open ended ending. We don't need a clear sense of what Lyra's future or Will and Mary's future holds, because Pullman is already going to apparently do that with future sequels.
Not knowing the future of a charter or characters always lets the reader do their own imagining of it, and it is much more satisfying than an anti-climatic and force-fed ending.