I'd originally planned for a comment, but then it got out of hand.
First, intelligence isn't transmitted that way. This odd sort of intellectual communism, where everyone has it just right isn't biologically possible. Even if you killed off or castrated the unintelligent and force the rest to breed prolifically, it wouldn't. Some names do spring to mind, but I won't name the people I'm tempted to kill here.
Intelligence sort of pops up unexpectedly, but more often, it's nurtured and grown, by the environment. Intelligence isn't going to be wiped out, but it is most definitely going to remain marginalized. Educated people, who are more likely to provide an education for their children, are also the kind who marry late and have fewer kids, so it decreases. From India to the bible belt in the US, statistics show this. So as the population increases, it's more often uneducated, people who have more kids, and the number of intelligent people does not decrease in a linear manner, and if your theory worked as planned, we would have drifted over to the far more unintelligent by now. But intelligence is hard to pinpoint, except for the Jews in the middle-east, who have higher average IQ.
Also, more people being at the same level just means they are going to disagree about the same things. You will just have less variety in terms of topics as well as arguments. There is no difference between Harun Yahya, and Kent Hovind(okay, I won't name all the names). Intellectually, they are both bankrupt, they are liars and frauds, but they have just enough neurons to keep their fraud going. Same level of stupidity, but still conflict will not die down. Or a better example, is Indian politics. Just the same. They all say the same things and are just about smart enough to keep their gig going.
Also, the idea of living in harmony with the planet won't really last, especially since the planet never wanted to live in harmony with us. Honestly, how long till the next ice age, when we all go extinct? The volcano at Yellowstone national park is about 30,000 years overdue, if I remember correctly. And when that blows, I don't think any of us want to be around. North America completely obliterated, rest of the world affected by tsunamis, covered in smoke, dust and storms. Or the next disaster that affects us. The environment will screw us over, a thousand times over, like it's always done. We will fail, since we've gone and evolved so much we won't survive the next great upheaval. We are still tuned for survival, but our bodies, are beyond the point of no return. Unlike cockroaches, who I still bet on. Override the programming I say. We aren't built to survive in harmony, we are a flash in the pan, and I will have my fun. Hedonism, is now the point of life for me, not survival, unless it gives you, personally, more time for hedonism. I'm not saying litter irresponsibly, or do things that will have an immediate negative effect, but please, don't talk about the whole planet as an organism, leave that to all the New Age people.
Hypothetically, we could mould the planet to suit our survival perfectly, but that would require psychohistory. But until Asimov's and many a sci-fi nerd's dreams are realized, we won't have any idea of what it takes.
Lastly, would you want a comfortable in between? I'd rather hope for the next Nikola Tesla, the next mad scientist who can boast of being able to split the earth in two. Or the next Frank Zappa, or Neil Gaiman. That is what makes being a member of the human species exciting, doing your own thing, doing the fun thing. We might destroy the environment, but hey, we're not the ones who asked for opposable thumbs.
EDIT: Ever since I put up this post, I've been dissatisfied with it. I might edit it later, or delete it, until I feel better about it. There is more that I want to say, but I don't think this is the right place.
EDIT: Ever since I put up this post, I've been dissatisfied with it. I might edit it later, or delete it, until I feel better about it. There is more that I want to say, but I don't think this is the right place.