Yes, we should start with the low-hanging fruit. For example, nutrients in human waste are a small fraction of what's in animal waste, and the latter should be easier to capture. Even so, much of the manure still gets applied at pollution-causing rates near barns and feedlots, rather than paying the cost of transport to where it is most needed.

But your point about food availability and social stability is more important. Recycling urine seems like a good idea. But a society that needs to recycle urine will be a society where many people are spending most of their income on food and others are going hungry, as was the case for the societies mentioned above.

stockpiling human urine to supply phosphorous for agriculture (as opposed to merely conserving it locally) is economically viable, that implies some pretty scary things about the general availability of food

Well, something like this has already been fake-newsed.