My English is mainly patched together from school+Netflix&HBO.. I only used proteins because it sounded right;-)
But you're right, I am more or less out of my depth from that point on. What I do know from watching and doing is how much of a difference it makes in agriculture to have good soil versus bad soil and also fertilizer and none. For example, in regions with bad soil and no fertilizer (I'm thinking about Central Asia/Afghanistan), people usually have goats roaming around, because they collect the thinly spread plant nutrient much more efficiently than it is possible to do with cultivating the land by hand. The detour over goat milk and meat becomes a necessary trick for survival and my point is that you can only avoid this by living in the right place or having enough technology/processing knowledge.
Well your english is quite good :-) "not enough protein" is one of the most common, though factually incorrect, arguments against veganism.
Absolutely, the soil (and microbiology in it) is just about the most important thing for the survival of humans, and every other living thing on the planet.
I would argue your last point a bit, it is quite easy (not simple, but with the knowledge, easy) to revitalize the soil.
Even in pure desert, we can create lush food forests: