You know, call me crazy but I think Andrew Stanton actually really thought about that opening sequence. I believe he may have had a specific meaning in mind, something along the lines of "if you keep throwing away stuff, you'll eventually run out of room." I even think he handled it relatively gently by introducing a dancing robot. No? He was all about flaunting his CGI skills? My bad. Sorry kids, here's a new toy to numb your emotional trauma. Throw it away when you're bored. No, trash doesn't pile up -- it turns into rainbows!
I'll freely admit that disaster movies can make annihilation pretty damn insipid, but complaining that Watchmen or WALL-E is irresponsible for showing devastation not only misses the point, but suggests someone is determined to live in a fluffy delusion where landfills don't even exist (let alone fill up!) and nuclear weapons shower us with lollipops instead of radiation poisoning.
For every 2012, there's a post-apocalyptic film (ones that don't star Kirk Cameron) that had a very good reason for ending civilization as we know it. (Yes, even Terminator.) If you're honestly too frightened to understand what it is in any given story, then stick to safer fare ... though I'm not sure what is left if Pixar and DreamWorks can't make the cut.