You have not yet correct answer to your question, which is not "the brain can contain all human knowledge? (the answer is not long long), but: "the brain will always be the possibility to acquire the foundations for pushing the frontiers of knowledge, in the short time of a life?
Must differentiate diversity and complexity of knowledge. The complexity is based on the number of levels of organization of the mind necessary to understand the concept. databases are used to understand an emerging notion harder, then it serves as a foundation for another even more complex. The spirit is not born with this stack of representations; He needs to build it; However this ability to self-organize is innate, best among some for the specific intelligences, while society pushes us to specialize in intelligence it seems to develop more easily.
Specialization is done quickly, before the age of 25 years, and is the true source of the appearance of new paradigms of knowledge. It is not so common. We don't not discovered each year an evolution of the species, a relativity of the time, a Big Bang, or the disappearance of the space in the quantum world.
The rise of the complexity pyramid has a side effect: diversification. Conceptual alternatives multiply considerably. Also producing a large amount of new, more complicated objects. It's actually this diversity that seems impossible to understand with one brain.
This has always been the case, but it was less apparent because the knowledge was not so easily available. She was even hidden, selected: a craftsman did not show its know-how to everyone, a scientist did not share its secrets with someone likely to steal his research. The vast majority of humanity lived with a limited amount of knowledge and knowing that there are many others, but they weren't his property.
The idea that he should know everything is a modern neurosis, because of course it is a goal that has always remained impossible. You hear even criticism on the fact that people use every day objects ignoring all of their operation. This prevents them to be happy? What is the problem from the moment where they have good representations of the functioning of the object? Critics might go instead to designers, who generally define user interfaces so as to force people to think like them, rather than in their own way owner.
If real progress of knowledge comes from the elevation of the complexity of mind and it already reached its peak around the age of 25 years, you see that it isn't the length of human life which is limited. Can this rise meet a limit? That is what defines?
See our brain like a gigantic mass of neurons/cubes, first all spread out on the floor, then who is auto-assemblent in a pyramid under the effect of the daily stimuli. Self-assembly is the sort of neural connections in specific patterns. When we are young, full of cubes are still spread out on the ground, without well defined role in the pyramid. To 25 years the pyramid includes all cubes in its structure. Subsequently it will be always possible to remove cubes and reassemble them to build new representations, but it becomes difficult to lower floors that threatens the stability of the whole. It's easier at the top. Change your mind. But here the psychological barriers appear: if specific thinking brings rewards, it becomes difficult to change. You have here the history of a researcher, whose intellectual maturation produces an original thought, then tends to get locked into this mode of thinking when he brought fame and awards.
The solution to your question now seems obvious: to allow the human brain to continue its complexification of knowledge, must not extend him his life but provide more cubes/neurons, encourage him to invent his own solutions rather than reward it to teach others, and possibly stimulate the cellular mechanisms that allow its self-organization , whose genetic springs are obvious.