First, it’s a catchy rhyme with 50/50, and 2050 is just too soon to be realistic. The point of the 50/50 by 2150 idea, at this stage, is to start framing conservation discussion in realistic terms. Is it realistic to categorize, study and understand every species on the planet and educate every human on the planet on how each species is to be conserved? Of course not, but CITES, the ESA, and many other conservation efforts seem to be driving in this direction. Is excluding (non-indigenous) human activity from 50% of the Earth’s surface realistic? Perhaps in 1950 it was not, but by 2050, the technology and resources will be available to make it possible. Satellite and aerial surveillance, infra-red imaging, low cost autonomous drones all make it possible to control a border, if you have the will to do so. It is much more realistic to simply seal a border than to police a complex, ever changing set of laws on a borderless population.
140 years ago, steam power and railroads had just started shrinking the scale of the world for the humans that used them. 140 years from now, those roads, rails, flyways, power lines, communication lines, pipelines and whatever else humans desire to transport and travel on, could be updated and relocated to preserve half of the Earth for natural systems, and much more easily than the current infrastructure was constructed.
Another point raised is: how will the “displaced” people work their way into developed areas. Again, over the space of 100 years, it is more realistic that the transition can be managed without unpleasant abruptness or glaring inequities. There is the problem of overpopulation, but population was at half of present (2010) levels in 1970, and half that again in 1870. The number of people the Earth can support is finite, and protecting 1/2 the Earth’s surface from advanced human development will not decrease the maximum number of people the Earth can support by 1/2.