Imperium Sine Fine?

Virgil calls Rome an imperium sine fine . Can he be serious? Every other city that appears in the epic — Troy, Carthage, Latium — is doomed. How can Rome escape? How has the world changed to make a permanent city possible? Perhaps I’m looking for too much philosophy from Virgil, but it is hard to believe that he would not have the insight to suspect that Rome, too, would fall.