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You also raise the important issue of whether or not "perspectives on the medium-long-term collective future of humanity would be more effective, in motivating high-school students, than the perspective of their short-term, individual, professional future". I think it could be, in some cases. For instance, future math majors sometimes do not see why they should know any biology, and students who want to pursue careers in the health sciences sometimes wonder why they need to learn about physics. If we can tie knowledge to its universal relevance for steering the future of humanity, I believe we can raise the motivation of, at least, some of the demotivated students. (Students who already love learning for learning's sake will do well no matter what.)
You mentioned Sabine Hossenfelder's essay, that suggests that we use "priority maps" to build a more-or-less automatic system that tells people how much their priorities match with the decisions they take, in an "intuitive and emotional way" that counteracts our laziness and lack of time to research the consequences of our actions. I found her approach very interesting, and it would certainly complement the education initiative I propose. It is useful to have automated helpers (like pocket calculators), but in order to use them at their full potential, I believe it is important to have internalized some basic knowledge about the way they work (you must know some basic things about numbers and math before being able to use a calculator). So even if one day we fully implement Sabine's system so that our decisions are guided in an effortless and intuitive way via a brain implant, I think the system will work better in conjunction with widespread future-literacy in the population.
As for the Millenium's Institute quote, "these great conversations will be better informed if we realize that the world is improving better than most pessimists know and that future dangers are worse than most optimists indicate", I know it sounds generic and somewhat vacuous, but I think it actually states something important. I know too many people who are concerned about the future of humanity, but who have so thoroughly internalized all the most extreme prophecies of doom that they fully believe that civilisation will collapse (never to rise again because of depleted resources), even if we outlawed all cars tomorrow and became all vegans. I think we need to keep a nuanced and balanced outlook, especially when we teach to young people who are just beginning to learn how the world works. (But we must also avoid to be overly optimistic and to believe that everything will turn out fine no matter what we do.)
Thank you again Tommaso for all the interesting issues you raised!
Marc
P.S. Your essay was one of the first I read, and I really enjoyed it, but I didn't take the time back then to comment on it and rate it. I will do that soon: see you in your forum!