I'm not saying that 100% of that 50% will be employable in this world. I'm saying that over time that 50% will inevitably become that bare minimum. The way I see it is 150 years ago, your idea would be that we couldn't possibly get all children to become educated at an 8th grade level, yet here we are, even making an HSD the bare minimum.
Eventually your masters thesis will be an area for you to study and pursue to make an attempt at furthering society.
Because that's the only way you get to post-scarcity.
This is a non-problem that has always taken care of itself in any developed country and we have no reason to believe it will not take care of itself in the developing world as well.
The UN for instance does not believe there will be 10 billion humans on earth ever (where "ever" means "as long as projections have any value").
The fact that we're cooking the planet with ~8B does not bode well for what even 10B looks like.
I'm saying that imagine 50% of the population works in blue collar general labor or semi-skilled labor fields. Now in this hypothetical worlds, all those jobs are managed by autonomous robots. Also we have a green power that is sustainable, storable, sufficient for even double the population, and can be held in high densities at low volumes. So there are now innumerable sectors within the economy that we don't need people themselves to learn. That leaves more time for people to take extended amounts of time to learn and study. I mean quite literally a Star-Trek "post-scaricity world" in a lot of ways. People use time to further themselves and expend time on cultural or scientific endeavors. Life is no longer about struggle and survival since money clearly would have no value if any and everything can be made or consumed for free. I mean it's really interesting to think that the only "conflict" that would exist is between people trying to min-max life in terms of achievement. There would be no achievement in religion, money, or ownership since everybody can do it.
Ultimately what I'm saying is that a lot of our advances are contingent upon other sectors becoming automated and allow for more people to get into academic sectors.