Actually, with increased density, some people benefit while others suffer. All non-human life suffers from human increase, but economic systems ignore that cost since it doesn't have a price tag. So, let's look at economics only as it affects the humans it's meant to serve. A large, expendable work force benefits owners, but it places labor at a disadvantage. Workers with dependents can't afford to hold out on strike, or take chances on being permanently replaced.
High unemployment reduces wages, while high demand for workers increases wages and benefits.
New housing provides jobs for construction workers and gains investors more capital for further development. With a shift in priorities, maintaining and improving existing buildings could provide as many jobs as new construnction provides. Businesses which focus on the needs of infants and young people will have fewer customers as birth rates improve, however, fewer customers doesn't automatically mean less income. Many potential consumers can't afford the products offered, and with smaller families, just as much money could be spent on children's products. Often, companies also cater to other age groups, and money not spent on children's needs is available for those products and services.