The assumption of the question is that, if such a city could never actually exist, then it would seem we are just wasting our time talking about it and trying to define its organization. Socrates replies by saying that the ideally just city he has described could never in fact exist, but he rejects Glaucon's implied inference that giving an account of that city is just a waste of time. The goal, says Socrates, is not to bring the ideally just state into existence, but to make actual cities approximate the ideally just city to as great a degree as possible. This text would suggest not. Socrates intends his account of the ideally just state to be interpreted not as a constitution for an actual city, not as a blueprint to be followed strictly in the construction of an actual social and political order, but rather as a standard or model to be used by actual rulers in deciding how to make some actual city more just than before.
Approximation is everything.
The important question is not whether the ideal city described by Socrates could ever actually exist, but rather whether his sketch of an ideal city could serve as a standard that actual rulers of actual cities might be able to approximate. This point would seem to be made clearly evident by Socrates' example of the painting of an ideally beautiful person. We could all agree that an ideally beautiful person could never actually exist. But that doesn't mean a painting of an ideally beautiful person would have no purpose. We could take the painting of the ideally beautiful person to the plastic surgeon and ask the plastic surgeon to do whatever possible to make our own features approximate those of the person in the painting.