Spoiler Alert! Theo's view vs a perception of a positive ending
So the implication of the ending (for some people) is that with Kee and her baby being saved that the future of humanity is also saved.
However, early in the film, Theo states (and I'm paraphrasing here), "It wouldn't matter if we started having babies now anyway. There's no world for them anymore." in reference to the global political-economic meltdown. Britain is civilization's last hold out and not for much longer by the look of it.
Sure, you may have built a house on the earth's tallest mountain, but God is still flooding the world and your house may last a little longer than everyone else's, but you're still going to be treading water along with everyone else soon enough. (so to speak)
So, for the people that saw a 'happy ending', isn't that just a self-deceptive illusion? A conditioned response without seeing the bigger picture? Of taking the surrounding context into account?
Aren't fertile women (Kee, Dylan, any other female children they have...) just going to be reduced from people to birthing chatel for the rich? Look at how the rich lived in Britain compared to the poor/middle class.