Because that's the standard ending for this type of storyline more times than not, done a thousand times already. I wouldn't even mind it, but the plot summary above describes their marriage as "two strangers, sharing a home" and the baby becoming the... glue that fixes something that's too broken, doesn't give me the best vibes.
Of course, I can't really judge until I see it. Besides, movie summaries are always a bit exagerating.

I didn't think about the possibility Kate could die, but now that you mention it - the clips do suggest that scenario is possible.