This will literally never happen. The labor force will look a lot different, the average person will be much more educated. But we are never going to be in a perpetual state of leisure, or anything resembling it, lest we want to halt human progress in place. All automation does is allow us to focus on larger problems, or create new ones.
What do you suggest we do with them?
Unless we can get to the point where we can manipulate genes to increase intelligence that's not a solution to the problem
As the high school coach used to say: "You don't need talent to practice!"
Barring people truly born with mental disabilities, anyone can do it with the right attitude and resources. I'd be less worried about the gene pool and more worried about the unequal distribution of education spending causing pockets of the country to be financially unable to educate people.
Do you have evidence of the contrary? Of the top X% of highschool students who make it to engineering programs at my university, roughly 40% fail or switch majors. If they can't do it, how do you expect average or lower people to?
It is simply not feasible.
How many scientist and engineers can any future workforce support? There will be a limit to how many "production" jobs there can be. By this I mean manufacturing, farming, infrastructure etc. This still leaves many creative endevours that can be produced and consumed almost without limit - but this also assumes people have expendable income.
As far as how many scientists and engineers we need? Virtually infinite if you're creative enough. There are plenty of problems left to solve that we haven't even started. The best motivator for finding and solving those problems? Money.
Look back on history and I bet all the things you think of as great achievements in human progress were the result of the leisure class's hobbies. Especially Math and Science!
Theres a darker reason why the majority of people won't be enjoying leisure- because the power to force people to work is actually a pleasure and a value of the ruling classes, and nearly everyone has internalized it.
Thats why you hear an outcry against welfare or basic income, but not a peep about trust funds or inheritances.