Whoa there, let's not put words in people's mouths. Gentrification has benefits and drawbacks, like everything else.
First, and foremost, the neighborhoods aren't always run down. The Missions and the Williamsburgs of the world aren't the only places undergoing gentrification. Sometimes they're just neighborhoods largely populated by immigrants (Greenpoint in Brooklyn), or traditional working-class neighborhoods (Potrero Hill in SF). Being populated by immigrants or working-class folks doesn't make a neighborhood run-down.
Second, what does it mean to "fix up" a neighborhood? I'd argue it's really just replacing the historic culture with the culture of the new residents (which is usually rich, mostly white people). Sure, one nice effect is a huge reduction in crime, but you're also often replacing historic establishments with rich people tropes (e.g. Murry's -> Whole Foods). Harlem will probably be a really safe place to be in 20 years, but isn't it going to be a little bit of a shame when the Apollo gets replaced with a coffee shop?