I don't think MBTI is going away. I think it's evolving. It has evolved since I first learned about it, and the last I heard, they still teach it to college students in psychology courses. Traditionally it is INTPs who build theoretical structures like this, which was first devised by Carl Yung, who was an INTP himself. But he only came up with, as I recall, Extrovert/Introvert, Sensing/Intuition and Thinking/Feeling. It was Myers and Briggs who added in the Judger/Perceiver layers. And even these days, these things are being expanded and evolved.
Until someone comes up with something better. People exist on a Bell curve in most aspects of life, but this seems to be something that the Bell curve ignores. Taking up Thinking and Feeling as "opposites" for example, seems to create a double-humped Bell curve, suggesting that these are two ends of a continuum, while a better view might be to see each existing on its own Bell curve. Thinkers feel and Feelers think, which we all are aware, there is likely another factor in our makeup or predisposition, or even our DNA which sets us to prioritize one over the other.
I think that the J/P preference is genetic. You see the Ps out there racing motorcycles and playing action games, while the Js are standing out of the way, be very cautious. I think if there is one thing that separates Js from Ps, it is reaction time. I believe that Ps are faster. Js, therefore in order to survive, tend to be more cautious. They might see Ps and being too reckless.
I have no science to back this up - simply observation and reading. But I'm convinced that this is genetic. And the only reason for these things to be in our makeup is survival. Ps survive by being fast. Js survive by being cautious. Here again is not a double-humped Bell curve, but rather a differentiation in our DNA. A P cannot become a J, any more than a J can turn him/herself into a P. There could of course, be many genes or only a few that make us one or the other, and perhaps tipping us extremely in one direction or the other. Or perhaps only part of the way. Some Js are more cautious that some other Js. Some Ps are faster than some other Ps.
I think that Introversion and Extroversion are dependent on our own sensitivity. Again, I'm convinced that this is genetic. We introverts tend to be highly sensitive to everything, touch, hearing, smell, taste, and sight. But we're also highly sensitive emotionally and intellectually. Again, each of these can be mapped on a Bell curve, but not one that is double-humped. Too many extroverts literally thrive on constant sensory input, loud music, loud people, constant touching, constant taste and constant visual excitement. Introvert are quickly overwhelmed by this massive sensory input, and this can only be genetic. Again, it comes down to survival, in this case, mostly of the tribe.
Introverts would not exist in this world, except that we help to keep the tribe alive. We're the early warning system. We smell, we hear, we see, the danger. The tribe dies without us.