It doesn't "trap" anyone; that's just hyperbole.
The main effect is that it gives a significant number of people a chance to stay in the city in a profoundly deteriorating market for middle-class housing stock (driven, in NYC's case, to no small degree by external factors such foreign investment, and radical and -- in the view of many -- essentially undemocratic "pro-building" changes to the local zoning laws).
this is one of the ways that ghettos come in to existence.
Perhaps. But given the vastly greater forces at play (racism/classism; the collapse of inner-city manufacturing base; and deliberate wholesale razing of neighborhoods in NYC, Boston and elsewhere; mistaken notions about the wonderful community-building effects moving people out of quirky single-family housing and into massive high-rise development projects; and just the inevitable effect of multi-lane freeways in high-density urban areas, generally) it seems you'd have a hard time pinning it as one of the major factors.