Darwinian natural selection doesn't select for absolute/universal fitness, because there is no such thing. It selects for fitness into a specific, local niche. Humans may be more fit than chimpanzees in some areas, for example, but less fit in others.
It is a fallacy to assume that evolution is linear, and that it progresses toward some sort of universal optimum. There is no universal optimum. There are only specific, local optima.
A good example is the megalodon. From all we know, the megalodon was basically a massive, faster, more badass, more ferocious version of the great white shark. Why did the megalodon die out, and the lines that led to the great white survive? Surely big-badassedness is a desirable quality in an apex predator like a shark, right? Well, not really, because circumstances changed. Ocean temperatures changed, prey got scarcer and smaller, and eventually, size became a detriment. Megalodon was too big not to fail. What fit one day no longer fit the next.
The same thing may happen to humans someday, and some other animal may step up to take our places. It seems unlikely, but nature is the one doing the "selecting" in natural selection -- not us. Environments and circumstances change, and as they change, so do the criteria that "fit" the new context. In the Darwinian sense, "fit" refers to suitability to a habitat; it does not refer to "fitness" in the modern sense of the word. Think "product/market fit," not "physically fit."