First of all, is something not viable at all unless it's viable in NYC?
Secondly,
Nothing but farmers and other rural people?
San Cristobal de las Casas:
Guess what isn't the main industry in one of the five cities that have autonomy?
Thirdly, it is difficult to gauge where and how anarchism could work in many places in the world, seeing how those who would lose power or economic standing in new circumstances tend to immediately oppress those people. Not many people realize that 8 million or so anarchists operated Spain for three years during the Spanish Civil War, implementing a working model of anarchist-syndicalism before finally falling to Franco, who was supported by Hitler/Mussolini, and they were simultaneously dealing with the Soviet Union funding other parties. After WW2, the United States help fund Franco's dictatorship for sometime.
If the question of viability is in reference to everything other than fighting off capitalists, I don't see anything that suggest that anarchism is not viable. If viability includes fighting capitalists, then that might be the one problem anarchism has... It's generally not a huge priority for anarchist communities to continuously militarize and strengthen the power of the military in hopes of expanding territory.
In addition, if a strong centralized government existed to create a military, then it cease to be anarchism.