OK. First of all, it’s important to realize that alleles are not intrinsically positive or negative within themselves. “Good” and “bad” are meaningful only in relation to the environment. Due to our modern control over the environment, we’ve managed to severely reduce the selective pressure against many of our alleles that were previously negative. Others remain selected against.
So what does this mean? Alleles that previously were negative and are now neutral are no longer constrained by selective pressure. Note that this does NOT mean that they will necessarily spread like wildfire. They’ll be more common now than they would have been, but they’re not going to rocket to fixation. They’re now subject to random genetic drift.
Now if we’re talking about “evolving ourselves to extinction”, that’s clearly not going to happen in our current environment. As always, we are evolving to adapt to the current environment in which we find ourselves. So if we are to become extinct, that would have to be caused by some change in our environment, so that we are suddenly severely maladapted for survival. The thing is, since this change is purely hypothetical, there’s little reason to assume that the same genes that were “bad” in our ancestral environment would also be “bad” in the new hypothetical environment. It could be some completely different trait that suddenly dooms us, but now we’re in the realm of science fiction.
It’s quite common even in well off families for orphaned children to wind up in the custody of their grandparents (see Michael Jackson’s three kids). It is also quite common in less well off families, there are many grandparents raising their kids today. In fact, for awhile my mother-in-law had custody of a great-grandchild. Lineages with long life provide a buffer for children who lose their parents, which is very important in a species with a long infancy and childhood, don’t you think? Kids do better with relatives than in foster case, all other things being equal (and sometimes even when they’re not). So longevity past reproductive age could, indeed, be a genetic asset.
Giant brains? No, that would be the eugenic answer. But we are going in the opposite direction, of saving babies that prior generations let die, the preemies and cojoined twins. That will widen the gene pool, not narrow it, and it will create a less-fit average person, but nobody will mind because of universal health care. We can keep the preemies and fix the the cojoined twinds and still live happily ever after.
And that “liquid diet” would be a pill, with a side of fake fats and sugar substitutes.
But males don’t really have a “reproductive age”; they just become somewhat less fertile. Anything that extends their life, extends how long they can breed. And given that we are the same species, woman will also probably benefit as a side effect.
“Survival of the Fittest” does not mean “the most physcially fit”. It means the most suited to survive in a given environment; and what’s mosre suited than a tool-using advanced culture making strides to actually overcome nature? We are the fittest (well, excepting cockroaches, perhaps).
We have no way of predicting the evolutionary process of civilizations more advanced than ourselves, since we’re the most advanced civilization we know of. But* social* evolution is an aspect just as important as any other, including our ability to combine resourcces, work together and beat diseases and injuries which would destroy members of another species.
I’d argue that we are still evolving. At this point, our evolution is centred around society and our ability to utlize tools; these are uch more effective evolutionary advances than the exceedingly slow genetic changes other species are doomed to rely on. We’ve actually out-evolved physical evolution. Our social evolution is a billion times better, and possibly (it could be argued) the natural evolutionary course of a species such as ourselves.