In theory, maybe. I wonder if it's that simple in practice. The hurdles to getting these things up-and-running today, for this many people, are substantial.

To my mind, the density of people in an urban area makes it unlikely that the available soil and trees handle the nutrient load. Cities, by definition, remove most of this capability in their construction.

The combination of soil liquefaction with its drainage issues, plus additional rain, start to raise public health risks.

The other wild card is access to the necessary resources. Cities, again, do not have masses of raw materials at hand. Straw, sawdust, wood chips, peat, charcoal, etc. -- these are needed to make composting systems work.

I think they'll end up with big collection bins for plastic garbage bags of human waste. That's what can be done in short order.

(BTW, thanks to the link to the tree bog. I hadn't heard that term. I guess my modified outhouse is half composting toilet and half tree bog. Next step is the wind turbine vent system, supplemented by a thermal wall.)