I think the internal tailbone we have is needed for comfortable sitting.
Broke my mother's tailbone when I was born, can confirm sitting is uncomfortable.
You are only going to increase the prevalence of that future generation to REQUIRE those advanced medicines, while the environment around them has no such requirements.
In short, you are going to architect a future where massive dieoffs happen when production of XYZ medication for ABC disorder does not ship on schedule.
But we create the environment. Forget about some hypothetical drugs that save lives in the future, I can give you that world today. The existence of antibiotics has done more to save people with weak immune systems then any invention we've ever created in all of human history, and yet I don't imagine you would support dropping those (maybe you do, who knows). We could cull a huge population just by not giving people the drugs they get today, but what would that achieve?
In fact you seem to self contradict. I will explain my reasoning, and I hope you clarify:
You at first seem happy to let evolution be evolution and cull the weak, citing your fear of a future where everyone is dependent on the same drugs and the fallibility of the medical processes. Later on however, you seem to criticize the gene-engineering, saying that taking any gene out requires limiting the diversity of the gene pool. Don't these contradict? You seem to focus on not interfering with evolution, but we've already done so by changing the rules by which we live. Evolution is a law of nature, not some sort of natural force that must be cultivated: it merely acts. We have surrounded ourselves with science, but we haven't hindered evolution, merely directed it through unintentional action. Evolution is content altering humanity to be more fit to live in the world they have created and are in, not the world they came from.
And I dispute the notion that evolution is a benevolent, or even positive force (natural fallacy). Evolution does not care that we exist. Evolution is not playing a game. If we were entirely wiped out by a meteor, evolution would shrug and continue with what managed to survive. The things that survived would be those things that are most fit to survive meteors: it doesn't matter whether it'll be a billion years until the next meteor, or a week, "can survive meteors" is what evolution has achieved and that is what it'll work with. But we don't have to do that. We might prepare, and survive the meteor, and by doing so prove that we are "fit" to survive meteors. Two things come to mind. One, evolution has no vested interest in any given arrangement: it's perfectly content to let us or any other species die off if it happens that we are not fit to survive, regardless of how "fair" or "representative" a risk may be (evolution didn't program you with the ability to survive nanobots? Dead. Is it true that you've never needed to do so until now, and would never need to do so again? Doesn't matter. You are a dead end because of nanobots, regardless of how perfect you might have been in other ways). Evolution is arbitrary. If aliens come to some planet that has only bacteria, and continually release a nutrient to organisms who follow some absurd and useless design, evolution will favor that design because it is no longer absurd or useless. It doesn't differentiate between "Survived some serious challenge to existence and prospered" and "Aliens fed me because they thought it was funny". Some other bacteria out there fought hard, but get's out-competed because of the aliens: evolution says "too freaking bad, should've been funny looking". Think of this using a capitalism metaphor: the market is uninterested in what works long term, it's interested in what works right now, and if that happens to match up with what works in the future then good; if not, it was good while it lasted. People profit off bubbles because there is something there that inflates them, and when it's not there, profit stops: that's the market working as intended. If our aliens disappear and take their nutrients with them, all those funny things die, and that's evolution as intended.
And secondly: evolution has no vested interests, but we do. Evolution does not care that her only intelligent creation is dying due to some absurdly specific disease: she expects the species to do the caring for her. What do I mean? I mean evolution is content leading us down some doomed path, purely because it works at the moment. But we can prevent it. And it is only because we prevent it that we are fit, not because of our ability to survive that doom anyway. Humans aren't good at much physically, and yet we have the right to be so large and populous because what we are good at mattered more then what everything else was good at. And if we weren't, we'd never have got off the ground. It is because of our abilities that we are where we are, and if those abilities failed us, we'd die, because those abilities just weren't good enough. Evolution has proved that intelligence is the right way so far, not mere physical health. We do not subvert evolution by saving people from disease, war, or starvation, we are acting as evolution has provided.
Summing up, your nightmare is already here wierd. Human population is already tremendously large, and it's supported by things that might well fail: government could collapse, markets could collapse, war, suffering, disease, anything. But clearly it works well enough, otherwise they wouldn't be there. The rest is up to the species' abilities: that means the fallible mind. That is why I support the guy with disorders that need medicine, but not the designer baby: saving someone is doing exactly what evolution has rewarded us for doing, messing with babies is something else entirely. But hey, it'll be alright: even in the scenario where it goes forward, evolution will cut people down, while the poor and middle class will still have all they need. We breed animals and plants, and we see that giving them all specific traits is indeed dangerous and unstable. Don't believe me? Try this: if there was some type of human that would really be fitter in every way then what we have now (as opposed to just seeming like it would be better: "A gene shown to increase intelligence? Let's shove it in"), why don't we have it now? And hell, even assuming that world still happens, regression to the mean and breeding between the classes would keep it in line. There are many, many ethical concerns to designer babies, but a nightmare scenario shouldn't be one of them.