That's not how biomass works, biomass is not a zero-sum game. The main issue with sustainable dairy farming is secondary elements and nitrogen fixing. My grandfathers both planted clover in rotation for this reason.
Eventually, the ground will become depleted of phosphorous and potassium. Other trace elements tend to take a really long time to deplete, especially in clay.
That was my point. What we see today in conventional farming is just what your grandfathers did "on steroids". We've become really good at farming, so the depletion happens significantly faster.
For every product that leaves the farm, it takes something from the ground. Some can be amended with legume plants, but minerals will deplete at some point - adding them yearly is responsible farming.
Eventually, the ground will become depleted of phosphorous and potassium. Other trace elements tend to take a really long time to deplete, especially in clay.