What would the astrologer have done if he had not left his village?
I have altered your question to read, "What would the astrologer have done if he had not left his village?" We know that the astrologer had to leave his village, and much of the story is about what he did. As far as what he would have done if he had stayed in his village, the author R. K. Narayan tells us the answer specifically.
He had left his village without any previous thought or plan. If he had continued there he would have carried on the work of his forefathers namely, tilling the land, living, marrying, and ripening in his cornfield.
He would have led a life like that of hundreds of previous generations of Indian peasants, but this unique individual was forced to flee to a big city and become an entirely different person. People who live on the land can survive without any money, but everything in the city depends upon money. The protagonist had to find a way to survive. He probably became an astrologer by accident when he got hold of the few bits of old "professional equipment" he uses to attract and impress passers-by. A peasant like himself did not have many options. He might have gotten work as a day laborer. He had no skills to offer. He might have had to beg--but there are so many beggars in Indian cities that he could have starved to death. How do all these other people survive?
Problems are often opportunities in disguise. The astrologer discovered that he had a talent for understanding and interacting with people.
He was as much a stranger to the stars as his innocent customers. Yet he said things which pleased and astonished everyone: that was more a matter of study, practice, and shrewd guesswork.
By the time the story opens, the protagonist is no longer a simple, superstitious, ignorant peasant. He has become urbane. He is a city dweller. He knows how to get hold of those little coins that mean so much. He has become relatively successful. Not only can he support himself, but he now has a wife and a child. No doubt there will be more children, and his responsibilities will be heavier. But he has evidently found a niche in which he and his family can survive.