So one little plant made it sustainable again? And then the earth is repopulated and all the living things return? Were they extinct (in which case they would not return). Hiding? What? What the hell is the ecology on this new earth?

I didn't say they left a lot of people to die, just that it's not explained. However 6+ billion people in ship(s) would be a little unbelievable.

The existence of the one plant shows that life can be sustained on the planet. I imagine it's not the only plant in the entire world, it's just the only one Wall-E and Eve found.

I would like to subscribe to the Suprmallet Ecology Newsletter that explains how one (or even a bunch) of plants, presumably the first in 700 years, makes a planetary ecology sustainable or even remotely possible.

one verified living plant means the planet is now a verdant, hospitable wonderland is precisely the basis for B'n'L's ridiculous (and ridiculed) evacuation plan. If I recall (on the memory of one viewing), the captain of the Axiom is presented with the obvious stupidity of this half-thought, when he has the ship's computer show him upon receipt of the plant the present condition of Earth: the same barren, virtually lifeless dump they had left. His conclusion (after an initial disappointment after the candy-coating of B'n'L's images) is not that the planet is finally, after all those centuries of patient waiting, ready to support life again--it's that simply waiting for that to happen is the kind of passive, lazy thinking that has stolen humanity's humanity and ruined the planet in the first place. This is why I don't really get the (apparently not uncommon) sentiment that WALL-E is some sort of anti-consumerist/environmental screed--the moral seems much more about the humanity and necessity of taking control of one's future, instead of abdicating it to lazy automation (and of course about the nifty irony of a discarded automaton re-sparking that idea).

In other words, humanity should never have left. It would have made much more sense to send the garbage into space than the people. So that lazy thinking was the result of the evil corporation convincing an entire planet of such idiocy. The idea that this could actually happen is, itself, idiotic. No matter how you slice it, the premise and plot are idiotic.