A couple of points...
1. I don't know what a "genuine sage" or guru or enlightened person is. Maybe you are referring to someone's standard or perhaps a standard of a religion or Eastern philosophy such as Taoism, Buddhism, Zen, or Hinduism.
2. I have never heard of the preoccupation with death that you mention. Maybe it exists among some teachers and their followers, but when it becomes clear what this life is and what lies beyond it then there is no preoccupation with death any more than there is with life. It's true, however, that there are teachings embedded in Buddhism, for example, that focus on death, the bardo, reincarnation, and so on, as part of its philosophy about karma and an ideal about a path leading from ignorance to awakening. But one who is awakened knows that there is no path and that all things of this world are inside of us.
3. When it becomes clear that only the body dies then there is no concern about mortality. This has nothing to do with denial or a belief system; it can be seen or known by having a perception that is outside of this system of so-called waking reality. Everything that exists has the same path of birth, death, and rebirth or renewal. This is as much a matter of science as anything else. In simpler terms, nothing lasts forever. This world is predicated on time and space that creates and destroys.
4. You posit the questions, "What would the world look like if we all suffered a near-death experience when we were young adults? Or how would societies change if we all constantly suffered a virtual near-death experience by having a morbid, melancholy fascination with our existential condition of being mortal? What would happen to selfishness, pettiness, tribalism, war, politics, economic competition, and crime?"
My answer to this is that not much would change at all. This is because it is the egoic sense that experiences near death experiences and therefore it may be affected in some way but the self is not dissolved. If the self remains then it continues living a petty, frightened, selfish, self-centered, angry, competitive, greedy, depressed, and anxious existence.
The main issue with this last point is that the self has psychologically fragmented away from the totality of existence or life. In doing so it has created the duality of the subject and object, the seer and seen, the thinker and thought, and the experiencer and the experience. Because these dualities are really nonexistent, the self has created conflict for itself and this personal conflict becomes the conflict of society. For this reason society and the contents of what we are, psychologically, has hardly evolved over millennia. Human technology has advanced, but not the human psyche and behavior.
