The "Pirate's Age" is all a huge metaphor for how empty and dissatisfying modern escapist fiction and materialism is At first, it looks like there's going to be a simplistic, black and white/good versus evil dichotomy between the dreamers (pirates) and the corrupted rule-makers (World Government and the marines). However, it becomes murkier when we're presented with less idealistic portrayals of pirates, and it becomes clear that even though they've done tons of evil, unforgivable things, law enforcement is needed. There's also some of the older pirates who were once cheerful dreamers like the Strawhats, but the clearest example is probably Mihawk, who achieved his dream of world's greatest swordsman, but is from what we've seen now a scowling middle-aged guy with no friends who picks on rookie swordsman for kicks. The One Piece is sought after, yet the clues of what it yields or whether it exists are vague at best, just a fun idea with no reality or substance. For most pirates, (but especially the young and naive) finding the One Piece is actually just a childish escape. For the main characters, the only treasures truly worth protecting are the people they cherish. However, Oda still goes out of his way to show that despite this, there are still miracles and imagination in the world, that life is wondrous and worth taking care of no matter how hard it might be.
The Enies Lobby arc furthers this idea, with its climactic battle a subtle deconstruction of shonen battle tropes. The two most powerful people go at it in a truly epic fight that, in the end resolves absolutely nothing. The only thing the two accomplish is preventing the other from accomplishing anything, and in the end both of them collapse near-dead at about the same time, while the day is saved by the secondary characters, in particular one who lost his chosen battle.
Lastly, Gaimon's whole character freakin' screams this. I mean, the higher-tier fighters in the world are often rich and tough, but none of them are probably anywhere near as happy and content as Gaimon, who's body is stuck in a chest and his best friends are Mix And Match Critters. He's being used to make a case for how a quiet family life can be preferable to one dedicated to seeking power and wealth, and that it's actually okay to be a "little person". Plus, even Makino remarked early on that fighting isn't always the answer, which supports the subtext that all the power struggles are ultimately only hurting the world.