Our current habit of disposing of crap sight unseen in ways that flush all those essential nutrients into the oceans may seem like a great improvement over the stench of the big cities at the end of the 19th century, but it is also entirely unsustainable.

Agriculture has gotten by with synthetic fertilizers for a while, but N, P, and K aren't everything plants need... they also yearn for the countless micro-nutrients in our bodily wastes which are lost when we flush. And while N (Nitrogen) and K (Potassium) aren't likely to run out any time soon, P (Phosphorus) is in limited supply and finding enough of it for our agricultural needs may become a serious problem later this century. Not to mention the fact that producing NPK fertilizer takes a lot of energy.

Hopefully most of us won't have to give up the flush toilet, but we absolutely will have find better ways of dealing with the "downstream" product and start recovering its significant value. In the not-so-distant future shit will be valuable again.