it seems, in order to achieve something, to make decisions, there must be priorities, which means duality. everyone may have all the same functions somehow, but everyone has unique priorities. so everyone gives 100% brainpower to a set of priorities.
another questions is: could type have been prevented, from happening? we have so and so much dimensions in our intelligence/worldview. or quadrants, as integral theory would name it. inside, outside, static aspects - substance - objects - facts (S), dynamic aspects - change - trends - influence (N), what is doable in this situation, how can we act (T), what should be done or made possible, could be a desirable action or situation (F)).
given that decisions imply specific priorities for those quadrants, we need to have all possible combinations of priorities, meaning all types.
the only alternative is additional transcendence, meaning every individual masters all possible combinations of priorities and has situational criteria for choosing one combination at a time.
i'd assume this is the goal of evolution, but it could not have been the start of it. we discover type, discover the validity of other possibilities of priorities, learn to understand each other better.
by recognizing/differentiating all these different aspects of reality, we had no choice, but to fall apart into different types of preferred priorities.
and we probably had to differentiate these aspects in order to understand time and tools, that means to prepare for difficult changing situations. advanced tools involve time, preparedness.