A BIG reason why the suburbs flourished is because of the automobile and white flight (these people didn't want to live near minorities and all of their associated problems). Thing is, these folks are not giving up their cars, and still don't want to live near minorities, or even *any* person from lower income brackets. They have no desire to move into Harlem and live near the longtime residents... they want them GONE. It's a predatory and opportunistic behaviour.

Gentrification wouldn't be as much of an issue if the new residents moved in and assimilated into the neighborhoods, but the reality is that --aside from the pioneering artists and hipsters-- they largely DON'T. Longtime residents are alienated. Attempts to remove them are increased, through blatant means as well as covert tactics like votiing for increased property taxes (or other taxes), using the police to bully longtime residents, etc.

Newly-created urban spaces would be priced and litigated as to ensure that "undesirables" can't move in. Meanwhile, these displaced low-income residents face the issue of where to live. They have been pushed/priced out of the city, and they aren't welcome in the suburbs.

Edit: One more thing: While these "gentrifiers" could instead be building NEW urban spaces, they have historically chosen the exact opposite. Plans to improve mass transit regularly gets shelved because "what about their cars?" New developments seem like they are built around control and access (HOAs, no sidewalks, police harassing people for walking, controlling access to public resources like parks, pools and athletic facilities, etc.) which is very suburban and anti-city in its' mentality. Living in a city means SHARING, something these people are very reluctant to do.