Creating habitat is not the same as creating species. The arks that left Earth will preserve some of the existent species. They would function more like zoos than a replacement terrestrial biosphere, even if placed into an advanced space habitat.
As to the alleged lack of an ending, as to whether humanity wins? We know that humanity will suffer massive losses due to the destruction of technology. Whether humanity will rebuild the old social system (aka technology) to resume massive destruction of habitat and mass extinction of species is unknown. That is a real example of ambiguity. Despie the many cliches about moral ambiguity or shades of grey around here, very few people are comfortable with ambiguity and fewer like it.
The scene with James Hong, where his official report condemns humanity yet still he wishes to die with humanity foreshadows Klaatu's actions. As he observes humanity he understands the unnamed observer's dilemma. Turning the woman and child into stepmother and stepson makes their affection from a sentimental fondness for one's partial (genetic) self, into a bond between two unrelated creatures. It shows a capacity for loving the other. Instead of telling Klaatu that humans can love other species the Bensons show.
Similarly, the action scenes show how helpless humanity is. Since it is true that most of the destruction scenes were in the trailers, how can anyone claim that the last third of the movie somehow reduces to ten minutes? Perhaps the impact was so great they magnified the time? Or perhaps their distaste for the story leads to saying anything critical, no matter how absurd?
Also absurd are claims about Reeves' acting---can't people read body language?, Jayden Smith's acting---loving a little angel is easy but loving a brat isn't, or Michael Rennie's acting---the sudden discovery that Michael Rennie was amongst the British greats like James Mason, Claude Rains, and---dare I say it?---Laurence Olivier is so absurd as to raise suspicion as to the sincerity.
And yes, "Klaatu barada nikto" is uttered to stop Gort when Klaatu is first shot.
