I'm kind of on the fence with your opinion. Living in Montreal I've seen some old broken down neighbourhoods being turned into new condominium cities, but without enough city/social planning. (Griffintown) This caused incredible problems for the local infrastructure, commerce, and services. Sewers, aqueducts, electricity, roads, public transports, kindergartens, schools, medical clinics, etc. The concentration of people increased too much, too fast.

Instead, I think we need to increase density slowly, but spread it out over the city. Not everyone needs to live in 300 sq ft closets downtown. Having smaller apartment buildings with 4/5 storeys replacing old duplexes and triplexes in adjacent neighborhoods, with units that are better adapted to family life with several rooms and enough space to move around could be even more beneficial. And include social housing mixed in with regular housing would have a positive impact as well. But, that's a pretty Montreal-specific scenario. I know in Toronto it's very different and their needs are different, for example.