I don't think this is wholly wrong, but there is an alternate explanation for why a small number of physicists advanced the field so much in the early 20th cent

I don't think this is wholly wrong, but there is an alternate explanation for why a small number of physicists advanced the field so much in the early 20th century while a ton of physicists don't advance it today. All the low hanging fruit has been picked. All we have left today in fundamental physics is the edges of particle physics and quantum gravity, and the latter has so far proven hard to research without convenient access to a black hole or a particle accelerator the size of the Moon's orbit. Convenient access to a black hole would be so cool we even made a movie about it (Interstellar). Outside fundamental physics a ton of these new physicists are working on important but niche problems: superconductivity, condensed matter, plasma physics, fusion, all kinds of materials science, etc. A lot of that stuff didn't exist at all back then, or only existed as a theory prediction or a proof of concept. A similar problem exists in other hard science disciplines. It's just easiest to wrap your head around in physics because the edges of our understanding are more clear. Soft sciences and humanities are by definition factional and open ended, so in that case more people just means more discourse. My point is that I think more than one thing is going on, and I don't think you can attribute stagnation to a single cause. When you make "then and now" comparisons, you have to look at the entire picture of what has changed. Single-cause analysis is almost always wrong.