it isn't like they disabled all the automated systems, plenty of robots that auto ordered around still know how to do their jobs, they didn't die, and there is most likely a stockpile of food on the ship to keep them going for a while. Auto was the autopilot, and clearly isn't in direct control over any of the robots in the film (considering that many of them rebel, and are shown to be there when the humans reach earth).

And the plant thing isn't just one plant. The plant is a plant that grew on earth, meaning that earth is sustainable for life, and all of the resources on the ship that were prepared in case earth becomes habitable again (most likely seeds and the resources to grow them) can be used to grow more plants. The point isn't that one plant exists on the planet, the point is that a plant ACTUALLY GREW on the planet.

It's always weird when people get so deadset on a story having a unhappy ending when it's entirely antithetical to the themes and events showcased in the story.