It's fair to suppose that in the far-off future, midrise high-density housing (the five-over-ones and texas donuts) built cheaply years ago would become some of the least desirable housing around. Unlike with detached housing, rehabilitating the structure is a package deal, so its quality will likely decline until the entire structure is demolished, likely in conjunction with a redevelopment of its lot.

The likely ossification of such midrises shows why incremental, small-scale increases of density, like allowing threeplexes on small lots, is so useful.