Might need to ship all the extra people somewhere. I understand most of the arable land is already used for that purpose by extremely efficient (whatever else you might think of them) megafarms.
Rob Greenfield's project only works because nobody else wants to forage the city and he says he farms yards of some people who don't do it themselves.
The problem with megafarms is that they are not sustainable long-term. Such intensive agriculture pulls nutrients from the soil faster than they can be replenished - year after year after year. So farmers have to supplement more and more with chemical fertilizers, and the soil effectively becomes little more than a hydroponic medium. Potassium and phosphorous reserves are a finite resource to be mined.
Rob Greenfield's project only works because nobody else wants to forage the city and he says he farms yards of some people who don't do it themselves.