One reason the “give me a classical field and I’ll quantize it” route to QFT has fallen out of fashion is its lack of generality and its tendency to mislead.

For example, classical electromagnetic fields are coherent states of photons. To get the quantum theory looking classical we just need a large N of photons.

But what’s the classical variant of an electron field? Think about why we can’t we answer this question satisfactorily.

Well, what do you know about photons? They’re bosons! But electrons are fermions and therefore can’t occupy the same quantum state.

So building a large N classical limit of them is far from straightforward and runs afoul of the naive “just quantize it” picture.