Welcome to permies Lukas.
From a large scale, commercial ag perspective, those farmers are correct.
How could you fit a 60 foot (20 metre) wide combine between the fruit
trees? lol
Commercial ag nowadays wants to put one single crop on any given piece of
land.
They feel that it is the only practical & economic way to do things - Huge scale - "Get big, or get out"
On a small scale, such as growing
enough for your family, livestock & a small surplus, I don't see how it could hurt anything.
Perhaps something like planting a winter wheat after the fruit has been harvested.
It should be ready for harvest next summer, before the fruits are ready to pick.
As long as you grow something that wouldn't need to be harvested at the same time as the 'main crop', it makes sense to get full use out of the land.
Anyway, how can you bake a fruit pie without the dough?
Good luck.