Physics is super old but for most of that time we didn't have the intellectual tools to do physics well. Once maths, engineering and the scientific method developed enough, the experiments were relatively easy to do (as the OP pointed out, Rutherford was able to do his experiments on his own) and the key results followed.

However they created lots of low hanging fruit. For example I would call relativity a monumental milestone, but the discovery that black holes are possible, the Schwarzschild radius etc were low hanging fruit made possible by Einstein's papers

Every endeavor has low hanging fruit. I assume the GP is referring to The Great Stagnation.