[djbebs]

Are you aware of the concept of risk? The amortization schedules are what they are for a reason, even if not all houses only last 30 years, the average is what matters here.

Any number of things can result in houses being destroyed and become worthless prior to that time, including damage from earthquakes, fires. Additionally poor initial building standards, or changing building standards, or new regulatory barriers can make certain properties become not just worthless, but have an active cost to their owners and therefore have negative value.