When Feynman was involved in physics the field was rapidly expanding and it would be a field that would attract truly ambitious people who want to have an impact.
When I got my PhD in 1998 the APS was projecting that 3% of us would get permanent jobs in the field. Some would say "the cream floats to the top" but in a world where I could get 30x the attention creating a web page than writing a paper, in a world where you could go into a much less competitive field (with more definite and rapid progress!) like Computer Science at the time, or where you could go into industry and become one of the founders of what was to be known as "big tech" or make your billions in finance I suspect the real cream didn't even start grad school.
The people who do get professorships can play a mean game of musical chairs, I'll grant them that, but the nature of the field is that someone who makes a major breakthrough in fundamental physics theory will probably wait 1/2 or 2/3 or all of a career to see it vindicated. For instance a major theory of how quasar jets formed from 1980 or so has recently seen some partial confirmation. The Higgs Boson was predicted in 1964 and observed in 2013 with a nearly $5 billion machine. Note this quote from Peter Higgs:
https://www.theguardian.com/science/2013/dec/06/peter-higgs-...
Neutrino oscillations were predicted by Bruno Pontecorvo in 1957 after he defected to the Soviet Union. He passed in 1993, oscillations were definitely observed in 2001 though we know retrospectively that oscillations explain an important experimental result from much earlier.
Another viewpoint is that Feynman and his generation solved all the easy problems and much harder ones are left.
Perhaps it is very different in some other fields. I look at biomedical papers today that blow me away in terms of how much progress has been made on understanding life in terms of chemistry, physics and such. Omics really had lived up to it's potential after a slow start. On the other hand, word is that it's even harder for young people to get established in those fields than it is physics.
