Another possibility is that there won't be one.

Oh, there are very smart people everywhere. But there was a lot more low-hanging fruit back then.

Einstein was able to explain Brownian motion, the photoelectric effect (for which he won his Nobel) and then Special and General Relativity, more or less by scribbling on paper in his study - long before computers were even a gleam in Turing's eye.

We might have run out of huge breakthroughs like this. There's absolutely no guarantee that there's an infinite series of Great Truths waiting to be discovered!

Certainly, it's very unlikely that anyone will be able to make a series of breakthrough discoveries in different areas in physics, because it simply takes too long to become expert in one field as there is so much more to learn than a hundred years ago.