The theme of the inevitability of conformity to society's norms and values is universal, as it speaks to most people's experiences. This theme is arguably the most important in Metroland by Julian Barnes. Despite his heady, intoxicating experiences of life amidst the Paris student riots of May 1968, the book's protagonist, Christopher, eventually, and somewhat inevitability, retreats back into the conformist middle-class world from which he sought to escape.
A quicker way of saying this is that Christopher has grown up. Metroland is a bildungsroman, or coming-of-age story, and so it's not surprising that Christopher should undergo a process of maturation. What is surprising, perhaps, is the fact that this entails a return to the stultifying bourgeois conformity of Metroland, which in his younger days approximated to his idea of Hell.
What makes Christopher's story relevant to today's world is that it chimes with the experiences of so many people in the West. Most people go through a period of naive idealism before eventually settling down into a comfortable complacency as the demands of work, home, and family start to dampen any enthusiasm for radical ways of living one may have had.
Barnes presents Christopher's gradual road to middle-class conformity in ambiguous terms. He doesn't present it as being inherently good or bad. On the one hand, he recognizes that there's something vaguely sad and rather poignant about putting aside childish things and going with the flow. But on the other hand, he gives the impression that there's a sense of inevitability about this process. In Western society, this just happens to be the done thing, and it's something that successive generations have done and future generations will undoubtedly continue to do. Whether that's a good thing or not is largely left up to the reader to decide.