Everthing in your body, from the phospholipids in your cell membranes to the phosphate groups in your DNA, requires phosphorus as a component. No life forms on earth can exist without phosphorus. Therefore you cannot say that it does not get consumed, since not all the phosphorus you get from your diet is defecated out. Now in traditional agriculture throughout the world, human feces was recycled into fertiliser for crops, and phosphorus was recovered this way, but in modern societies all the sewage is flushed into the ocean. Most wealthy westerners would not eat crops fertilised with human feces. Moreover, the current world population cannot be fed without mining mineral phsophate for fertiliser.

Nitrate fertiliser cannot exactly be said to be "created out of thin air" since although the nitrogen is indeed extracted from the atmosphere the Haber-Bosch process requires natural gas as a reagent, not merely as an energy source. Even if a magic source of infinite energy were to appear, you would still have to develop a means to replace natural gas in production. Now, there is much research being conducted on this issue, but it will be many years before any commercial applications will ever be seen. Do not be unrealistically optimistic, we are not living in India.

I would refrain from projecting human population that far into the future. And even if the population were to decline, once people become wealthier they will demand a higher standard of living and therefore consume more. I read a few years ago the average American consumes four times the resources of the average Chinese (maybe the gap lessened as China prospered recently) and something like 250 times that of the average Ethiopian. Chinese should not turn into American Amerifat