It's all fine and good until you realize the fit, high income, high education types have like 1 child for every 3 that the rest of the population has, and maybe what...1:5 for global? (Insert real numbers, I don't care to look them up).
Smart, educated, fit - these are not necessarily population fitness criteria. Just as non-religous may not be. Despite it being irrational, it may have better fitenss characterstics. Dying 5 years younger may have no impact or even a positive population impact, if reproductive age is significantly lower, etc. Just look at our welfare system, it's crashing under the weight of low birth rate
Or to put another way, humans seem to do better as long as they have a critical mass of inventors/thinkers/visionaries. EVen if the population kills them for their blasphemy, a generation later they all adopt it and forget they ever murdered the inventor, and they carry on all the stronger. If you're at the top of the food chain, the "ideal human" spectrum, whatever, the bell curve ensures you will be lonely there...
We don't understand a fraction of the natural world. We feel so wise but we're more like a symbiotic life form riding on top of a relatively gigantic "human body", composed of trillions of individual living organisms that do things way beyond our understanding, without our consent, and have for millions of years. And how do we use that culmination of billions of years of evolution, of the almost infinite expansion of our universe? Masturbating. GOing to church. PLaying a computer game. The billions of cellular interactions and DNA being stranscribed and error checked, the immune system battling millions of foreign bodies day-in, day-out, and that's what we use it for? Texting gossip?
We're lucky nature isn't sentient. It would spank our ass for being so dumb!
(If nature were sentient it would likely be making the similar mistakes...of course I know this!)