This was a big issue for me. When I was younger, I always had in mind an "Amish paradise" as the new system, really a decentralized agrarian society. Really, this is the only type of society which would be realistic under the JW constraints of their own "new system," since it allows everyone to struggle along at the same level, relatively speaking. But then I started thinking about cities and their socioeconomic structure and studying the nature of economes (eventually getting me into my current major here at college) and realized it would be impossible to not have an economic system, because human economy is as natural as nature's ecology. In fact, it is, in essence, human ecology. The JW new system can't account for the division of labor and an agrarian society to equal degrees. Society would gravitate to villages and cities, leaving some stragglers behind, just like what happened 150 years ago, simply because we have all the knowledge to make our lives infinitely easier, and human nature would take advantage of that - regardless of any ideals of an "Amish paradise."
Another problem of having billions of people living in a decentralized, agrarian society, spread out over the landscape, each eeking out their own subsistence without thinking about the regional and global effects of their resource use, is environmental degradation. Just look at Rwanda, Malawi, or Haiti in particular, to see the devestating effects of deforestation for mere fuel, much less the pressure on simply having enough land to grow food for your family. Rwanda was at one time one of the most densely populated countries in the world - and they were largely an agrarian society. Now throw in the added complication of population growth without a ~1% death rate and you have a few problems.