Public school education will disappear because it had lost its social unifying significance in a new entertainment culture and economic realm.
It's a method of comparison like "If it is true, what will happen with that or that". Postman tried to predict through assumptions that everything happening is defined by consciousness, values, purpose and the ability to think. He put his own field "education" in a context of new culture he had observed and researched before and tried to estimate the possible impacts on the education trough the values and purposes this new culture provides. He believed that schools' primary social function is to create a common culture among citizens through the communication of unifying purpose-giving narratives rather than to simply initiate children into the economy.
Postman, Neil. 1995. The end of education: redefining the value of school. New York: Knopf.