The answer is that Smith saw the all-important division of labor as a once-for-all, not a continuing, process. As has been recently pointed out, he did not see the organizational and technological core of the division of labor as a self-generating process of change, but as a discrete advance that would impart its stimulus and then disappear. Thus, in the very long run the growth momentum of society would come to a halt—Smith once mentions two hundred years as the longest period over which a society could hope to flourish. Thereafter the laborer would return to his subsistence wages, the capitalist to the modest profits of a stable market, and the landlord alone might enjoy a somewhat higher income as food production remained at the levels required by a larger, although no longer growing, population. For all its optimistic boldness, Smith’s vision is bounded, careful, sober—for the long run, even sobering.
source
Robert Heilbroner, The Worldly Philosophers 7th ed. (1999), Chapter III. The Wonderful World of Adam Smith