I think the argument isn't "engineer" vs. "farmer", but rather engineer (or doctor, interpreter, commercial farmer/farm manager, industrial project manager, any other specialization that realistically requires years of training) vs. lower-skilled labor like farmhand, non-management/unspecialized construction worker, stuff that can be taught and learned relatively quickly.

I wouldn't call "low-skilled" workers _unproductive_ per se, and personally think they're incredibly valuable, but economically, the cost/difficulty of replacing a "low-skilled" worker is relatively low: it's a lot easier to find a replacement farmhand than it is a replacement farmer that manages the farm itself.