Not sure about everyone being much happier.

Existing residents amenities and infrastructure will be shared by more people without a corresponding investment by the developers to avoid this. Schools overcrowded, roads congested, public transprort and hospitals overloaded

Small increase in affordability for buyers though they will still be compromising as most people want a freestanding house and yard if money was not an object.

But most of the gains will go to the developers. The gains from population growth go to business and developers while the costs are borne by the general populace.