The externalities have come back to bite society at large in numerous forms. The unmanageable maintenance debt on the infrastructure to support them. The dependence on private enterprise storefronts to supply developments that can close up shop at any point and leave whole regions without access to a proper grocery store. The ongoing maintenance cost upkeeping all the land and structures. The time lost commuting around areas of extraordinarily low population density. The structural macroeconomic expenses in building supply chains to support such a spread out population. The social isolation and resulting depression that plagues the society as people silo themselves off behind their picket fences.

The broader trend must be towards density. That doesn't mean to compromise privacy, or even necessarily individual usable space - but it does mean a move away from stick houses with 4 lanes in between towards 4+ stories with bus and train access.

Terraced housing as seen all over Europe could house single families without issue. London is full of these things.

100sqm is really quite a decently sized home with gardens front and back. That's already a 10x density improvement.

Not everyone wants to live in an apartment surrounded by other people on all sides with no control over the place they live in (because it's all outsourced to a "management company" e.g. nonsense bureaucracy).

As far as I can tell all you're going to do by building tons of flats is entrench some sort of class system. Outside of elite city center locations only proles will be living in apartments.