I don't find it anti-climatic at all, it ended on a strong statement and in line with the rest of the manga. One might not agree with the statement, but it's far from a blurb or montage of a vague happy ever after or something would would be anti-climatic. Now she's basically a sex maid until she disintegrates, and has no recourse in society at large due to her infamy. So she's stuck with Shuu who proved, albeit less of a disgusting human than a lot of the previous ones, one all the same with no real alternative.