I asked a grass fed cattle rancher that question and he just said thats a hard question, but also grain fed will always be cheaper because they use a breed that can about grow twice as fast/large. I doubt those steers could get the same calorie intake from grass feeding. Which is a separate question from “if we used all land available for cattle grazing how much grass fed beef would we max out at”? I suspect not near the level of demand for beef unless we also took back huge areas of crops and forest.