If I could go back in time I wouldn't have watched this

First, some credit: I'm sure many of us were asking how these unarmed, zero tech animals who were on the brink of extinction by eating all of their only (??) food source could travel through interstellar space. I thought of the eventual ending about one quarter into it, and was impressed with the logic they revealed towards the film's end. BUT, why did it not occur to anybody until his wife came up with it, that they could wipe them out with our 30 years worth of stockpiled toxin, rather than shipping it back to the future with its barely half a million outnumbered souls to do the job?? We knew where and when the aliens first appeared, we'd have the entire military of the world waiting for them, they wouldn't have had time to breed prodigiously (as I assume they must have). Mission accomplished, and all of the social upheaval would have ceased with the good news of the salvation of our future. Hell, it might even have united the world as we worked against our common foe. And the best minds of the world couldn't see what Chris' wife saw almost immediately??? I won't go into how these films always use bullets, but never a hand grenade, to kill off the aliens.