Maybe the amount of geniuses grew proportionately, maybe there are more geniuses per billion capita today.
My guess is that there are vastly more geniuses today but that they are spread out more geographically and to many more types of art.
Today, maybe Beethoven would be making techno in Rotterdam. Goethe might be making movies in Hollywood or computer games in Copenhagen. Who knows.
Still, culture operates on a "hubs and authorities" basis -- there are people whose job it is to be hubs (curators, publishers, music programmers and promoters) and people who become known for their genius ("authorities").
The interdisciplinary issue is interesting. There's probably enough of it happening to offset fragmentation of culture into specialisations.
Funny that you don't even consider the third alternative of there being _less_ per billion capita today.
You could argue about the dialectics of modern technology - did earlier geniuses appear because of the lack of it? What would Einstein have achieved with access to computers? Maybe he would have never relied on his all-important thought experiments...
Maybe it is just interests of people shifted from classic music and philosophy to other topics.
But I found contemporary piano music good in my taste: https://www.youtube.com/watch?v=7maJOI3QMu0
With 7.6 billion people, greater access to education, and more travel, how many meetings of comparable people happen today?