IMO "abundance of low hanging fruit" is the main factor As Physics got more saturated, and matured, these geniuses somehow just stopped.
...but I'm not convinced any of them are low-hanging fruit. Almost everything on this list would require sustained R&D across several fields of expertise, and many of them have the significant added challenge of overcoming entrenched interests / power structures.
That's not to say we shouldn't try! Solving even a small part of anything here would materially improve quality of life for a lot of people - but these are not "I could solve this in a weekend" problems; these are generational problems, towards which meaningful effort could easily be measured in thousands of person-years from entire teams / organisations / ecosystems of smart people, and where even a chance at success would require a steady supply of adequate resources that is durable across periodic economic shocks.
CERN is the closest thing I can think of as a useful mental model here: sustained cooperation across multiple nations, massive amounts of public investment, clear tangible outcomes over a period of decades.
Perhaps it requires genius?
But there don't seem to be as many, or as much of a percentage of, people wanting to do hard work either.
You've landed a great list of worthy causes though, respect.
The geniuses can still make very valuable contributions to humanity. It's just that the general public will have a hard time understanding a new achievement. We already see this everywhere. "Higgs Teilchen" and so on. The media showed it, nobody cared and everyone was confused (found at the CERN in geneva). It only makes sense.
We still make immense progress, but in very detaily branches often, that people just want to use but not having to know about, because of the sheer complexity.
I wonder where the borders of humans will be. :) We might not be far from them anymore. Nowadays, to get into unknows territory, you need to do scientific research for decades often and an exceptional brain. Because you need to get the basics first, to which others created the paths.
The next real innovation I see will be affordable space travelling and populating other planets. Other than that, some minor stuff will be implemented like nuclear fusion reactors and the like.
The hardest thing will be to define "individuals" who contributed some of that stuff alone, as it used to be in the past (more or less).
That's what the idea is. Once someone has figured out the basics eg energy conservation, relativity, etc, what's the next guy going to do? Put a small wrinkle on it at best. So the detaily thing is actually not immense progress, it's some small modification or application to a niche area.
The closest equivalent I can think of these days is Satoshi Nakamoto - assuming it's one person. But even their discovery is in a subfield of a subfield.