Then you missed the point. It doesn't matter whether PageRank was something trivial any grad student could come up with or if it was on the level of Einstein's General Relativity in difficulty. The point is that it was one basic idea.

Today if you want to dethrone Google you need to overcome the enormous amount of engineering that has gone into Google Search and Maps. You can't do that by just "discovering" something as a grad student. It's going to take thousands (or millions) of engineering hours to achieve.

That is what it means for the low-hanging fruit to be gone. It's like the difference between discovering electricity, as we all know took quite a while but was achieved by a small number of scientists and inventors over a period of a couple centuries, and trying to compete against the modern-day electrical distribution network on your own, which is essentially impossible without some kind of Star Trek alien galactic empire level technology.