If you destroy the "weak" then you're out of a workforce and then you'll have to spend a lot of your time merely making sure you survive. They are basically slaves who think they have freedom. Also I certainly wouldn't want to see IQ be a basis for who survives or not. Firstly because you can practice IQ questions, a lot of them are very similar; and secondly because you get smart assholes as well.
I agree wholeheartedly, destroying the weak would leave most trivial tasks to those with much higher skill and ability, speaking in terms of modernity... We need these people to maintain socio-economic stability today. However, due to the overwhelming amount of inept, incompetent people... even these jobs are disappearing.
In reality, we don't need a workforce. We need self-sustainability, people of strong character and intellect for any sort of healthy proliferation towards the future, but like you said, there are tons of smart assholes in the world, but I'd still rather have a smart asshole in my face, than a dumb, self-righteous asshole... either way, IQ testing should be relative to the person, they would need to exhibit some form of useful skill or trade needed by civilization as a whole, else if.. to the grinders.. by then, people will forget about cell phones and playing video games, and feeling important about themselves... They will be too busy mastering a trade.. there would definitely be less interest in lackluster masturbation activities (drug abuse, television, mindless music) and more impacting actions and lasting effort in the world. (active within a community, promoting healthy lifestyles, quality art.. etc)
I'd like to think of it as snipping the dead or weaker flowers from the rose bush so that the ones of highest quality can grow and strengthen... Why should people matter anymore than a flower? Where does this writ of sacredness of human life come from? As Conservationist said, we are not precious snowflakes.. we would all like to think we are, this way nobody is disappointed when they realize they are worthless and will succumb to either mopping the same floor on a daily basis or collecting a monthly 'disability' check, because they are too lazy/stupid to hold down a job, and becoming a leech on the neck of society.. Modern ethics support these types but, if you are worthless.. those are basically your only two choices. So we know where to begin with the grinding.