When a family splits, that means 2x everything (e.g., sofa, dishes, TV, etc.) Same goes for the social "norm" that adults above 25 y/o (?) should live alone (if they're not married).
The issue is, not only is this counter to long-established human (emotional) needs, but it is resource intensive as well. Put another way, we're bringing Mother Nature to her knees, and we're too depressed about ourselves to care about her.
What could go wrong?
People (read: their egos) are their own worst enemy. If you resist the status quo and "the Jones" it's not as bad. Perhaps not the same as before, but not as bad who tend to live beyond their means.