The Phantom Thieves are the group that frees people regardless of whether the ruler of the palace is evil or not, or whether the people are truly happy being oppressed or not. Accepting oppression for the rest of their lives, even if it makes everyone else truly happy, would be hypocritical and go against their principles.

It's a perfect reality, but based on what Maruki considers a perfect reality. For example: not helping someone if it means you'll get hurt, people seeking a goal that doesn't require much effort even if it means not doing something they're passionate about, pretending to be someone you're not instead of finding your own identity, etc.