I could be totally wrong, but my hunch is that this is more related to the lowest hanging fruit rather than human capacity. Maybe scientific discoveries simply get exponentially harder. In painting, it's very rare for someone to make anything original these days. We won't have any more Picassos, etc, simply because they were the first to explore a new terrain. It's really not a matter of education, genetics, or talen…
If worthwhile knowledge is in a "fractal" search space maybe there's hope. Those kids can still have a childhood and discover something new by the time they're 30, something worthwhile.
Another problem with modern genius might be that there are plenty of geniuses, but the proximity of their deep-in-the-fractal landscape is so far from what can be recognized and acknowledged by gatekeepers of genius sainthood that their work never comes to light. There also are probably lots of geniuses doing outsized work at relatively menial jobs in places where nobody can see it, and their work is taken for granted.