Complexity = elementality structure
The ancient Greeks did not know what modern physics knows, that structure also determines the properties of complex wholes even when they are composed of the same elements, and they did not know about abrupt phase transitions. Here I must exclude Heraclitus, that clever mtf, who introduced the transition of quantity into quality into the laws of his dialectic.
However, they correctly concluded that the complexity of nature cannot be explained without the existence of elementary properties - the elements of nature. Pre-Socratic philosophers had a rather naive idea of the elements of nature, which they classified, like Empedocles, for example, into earth, water, fire, and air. Then Democritus appeared with his atomism, which attributed the elementary properties of nature to the smallest, further indivisible particles of matter, which he accordingly called atoms (atomos = indivisible in Greek).
Therefore, Democritus' atomism had an advantage over Parmenides' monism in explaining the complexity of nature, while Parmenides' monism was at the forefront in establishing the unity of the material continuum with the continuum of space and time. In this sense, general relativity is the modern realization of Parmenides' monism, while quantum field theory and the Standard Model are the final, mathematical version of Democritus' atomism.
It is also interesting that after Democritus, Epicurus realized that atoms could not interact through Democritus' void (empty space) and therefore attributed to Democritus' atoms the ability to randomly deviate from a straight path - an indeterministic "clinamen" that allowed them to approach each other and connect into complex wholes. Therefore, indeterminism is not an "invention of 20th-century physics", it is an idea that Epicurus came up with in the 3rd century BC, drawing conclusions solely on the basis of Democritus' corpuscular concept of matter. Accordingly, QFT & the Standard Model proved Epicurus right in that the 17 fundamental particles (elements of nature) and their interactions cannot be explained without indeterminism.