You forgot about the height. It will overshadow the smaller crop, and lack of sunlight on this smaller crop may simply not produce any ears as a result. Just a "balcony farmer" but that is what i have seen with most plants that are overshadowed by dense/taller plants.

That’s true in generation 1, but breeding isn’t a single generation. You repeat things across generations until you find a stable mix.

You just raise the head.

You harvest the top crop, the one you want, cutting above the lower height, then go back through and make a second pass with the head lower.

You'll lose some of the lower crop, crushed under the wheels, but it was there as bonus anyway.

Short grains are modern;

- allow modern machines to harvest it more easily

- move slighly more of the plants energy into the seed

- prevent wind damage. (small farms bordered with trees was more typical historically)

On the historical side;

- The long stalks allow the crop to be bundled and dried without the seed touching the ground.

- It makes the stalks themselves useful as a building material in walls, roofs, isolation, bedding.

- It makes for a good grip as the seeds are pulled form the stalks with an historical thresher.

In short, we didn't start breeding grains to be short until modern post industrial farming techniques. Short grain does provide better yield on industrial mono-crop farming. But would have been seen as worse historically.

They confer advantage over many weeds according to the article.