I would challenge the assertion that the average person 100 years ago would be a "genius" today. Firstly, you have no sources to back that up. Secondly, it's false.
Let me introduce to you the "Flynn Effect."
James Flynn invented the standardized IQ tests 100 years ago. Researchers have been testing people all over the world since. What they've documented is that every generation worldwide has scored higher on average than the generation before it. Today's average person would be considered a genius 100 years ago... literally. The worldwide IQ is more than 30 points higher today, on average, than it was in 1920.
The caveat is that since 1975, the developed world (US, Europe, and some Asian countries) has slowed down, and recently some countries have declined slightly in average IQ. Norway started leveling off in the late 60s with ups and downs from year to year. American tests dropped for this first time around 2005, but only a fraction of a point. But still LEAGUES above 100 years ago. Meanwhile, developing countries' IQs are still climbing at a rapid rate today due to technological advancements, improved nutrition, better educational opportunities etc.
People interest or disinterest in baseball has nothing to do with intelligence. You could argue it has to do with attention span, something that is getting shorter on average for Americans due to modern technology and social media. Or you could also argue it's because the horrible umping, unwritten "rules," and crusade against anything fun among other things. You could say the game isn't evolving with the interests and entertainment preferences of modern America at a rapid enough pace. Still wildly popular, so I don't see it getting to the point of NASCAR, but something has to change.