The concept of the over-promoted society where general intelligence has been declining since around 1800, and where the average person from 100 years ago would probably be in the top 10-15 percent of the modern population, provides an explanation of what happened to the geniuses.
Because, looking around the intellectual world, there seem to be approximately zero geniuses.
(At least it is apparently zero if the criteria of the past are applied.)
In intellectual history it is interesting to observe how different subjects dominate at different eras; and how genius tends to migrate from one area to another.
For example, one of the last eras of genius was biology, especially genetics; and a significant number of the biology geniuses had migrated from physics - which had been the previous dominant area of science.
From this I assumed that the decline of genius in biology would be accompanied by a rise in some other area - and I was continually on the look-out for where this might be.
My first idea was computing science; but it seemed clear that the breakthroughs had been made several decades ago and the field was no longer alive.
A second idea (please don't laugh) was economics; and I read a great deal of economics in the mid 2000s - partly to see whether this was correct.
However, I realized (from about 2007-8) that economists as a class lacked basic honesty and were not motivated to know the truth. Any impression of genius in modern economics was mere public relations, hype.
I still kept scanning the intellectual horizon; on the assumption that there must (surely!) be geniuses just as there had been for 100s of years, and they must (surely!) be doing great work somewhere.
But this was challenged by a careful reading of Charles Murray's Human Accomplishment which made clear that zero (detectable) geniuses was the norm in world history - and that the distribution of genius across time and space was very uneven, and shown no tendency to equilibrate (fill the gaps).
Of the near geniuses/ unacknowledged geniuses whom I knew, only one was successful in career terms (and that after their scientific apostasy) while the others were grossly under-promoted and indeed persecuted by academe.
So at that point I thought that there were the same number of geniuses as for the past few hundreds of years, but that they were held back and not recognized; and therefore they failed to have impact.
This is clearly correct, and genius is discriminated against while various types of mediocrity are promoted and celebrated with moralizing zeal.
But it is not likely that all geniuses of the past were able to fulfil their potential, nor is it likely that discrimination would be able utterly to crush genius had it been as numerous and vibrant as it was a century ago in the West.
My current view is that the age of genius is over - and the West has returned to the normal default state for humankind.
That is, genius is now a very infrequent occurrence; and while it may have a significant impact, it does not change the fundamental nature of society because each genius is isolated and the breakthroughs generated are not frequent enough (in the same domain) as to alter the way society as a whole functions.
Furthermore, genius is now 'misunderstood' in the sense that the general standard of intelligence is too low for their work to be comprehended; so in fact the work of a genius (those rare isolated instances) can seldom be acted upon any more - and therefore the fact that there has been a genius is not longer obvious.
What we have, then, is that the decline of intelligence means there are many-fold fewer geniuses in the West (ten-fold, twenty-fold fewer? Or an even greater decline?); plus that those geniuses who are born are less likely to get into a position to make a significant contribution due to Leftism and bureaucracy; plus, even if they do excellent work and make a theoretical contribution, this cannot be recognized (because their work cannot be understood by enough people) - and they will not make a significant practical difference.
So there are now many-fold fewer geniuses; and the few there are, are invisible. And even if not invisible, they make little or no difference to society at large - because modern society is incompetent to use the products of genius.