High density neighbourhoods solve many problems when designed to be self-sustained. The idea is that residents barely need to use their cars when going to work or shopping. However, if the high density community contains only residential development where residents need to exit that community to get to work or to shop that is when high density may cause more congestion than it solves. That's why Mississauga needs more downtown office buildings while the getting's good.
In large part, a city's reputation rests on its central core, with a decayed and hollowed out inner-city tarnishing a community's reputation (even if it may have clean and affluent suburbs) and a healthy city core being a source of civic pride that encourages tourism and new migrants to move to the city.