Does Density Make A City Thrive?

"According to the pro-density argument, urban institutions require a certain threshold population to support them. If not enough people want to shop or eat out, there won't be many good stores or restaurants. If the audience for music, theater, or art is small, these activities will not flourish. If the tax base is scanty, schools and municipal services will be substandard. Even parks need people to use them, and if the parks are deserted, they will not receive the upkeep they need to remain attractive... If density is such a good thing, why haven't more people gotten the message?"