I think you're building a house of cards with highly leveraged, cash flow negative properties that will at some point collapse.

When they rot they will create voids that will cause the foundation to collapse.

Perhaps I am playing it safe since we are newer to the game or perhaps I can see a fairly large economic collapse happening in the near future. 

Although I don't think a collapse is imminent, I'm buying properties "off market" that have equity just to safe.

For older rentals this is something I would love to have a plan for, should the water ever stop draining due to a blockage/break/collapse  (or worse- a backup into the house) of an old underground main sewer line. 

Short of catastrophe - i.e. the economy collapses so hard that people are living under the bridge rather than paying for a rental because they can't - you are always going to find people that would rather live in a house than live in a cardboard box. 

It has been observed across the nation in the last 30-50 years that most injuries and deaths caused by earthquakes are caused by things falling on people (not structural collapse).

I think an underreported issue is that the prime first time homeowner demographic are 'children of the housing collapse', who grew up watching their parents and friend's parents lose their homes or be stuck with negative equity.

For the first 3 years after the great collapse all the turn key guys went to promoting IRA buyers.. all cash 60 % of all houses are bought with cash..

Unless the wall is literally on the verge of collapse, you should be able to stabilize it with an engineered solution and a lifetime transferrable warranty against further movement.