Nationwide, if you are going to use the argument, "he loses nothing unless he sells" then you also have to say "he has not gained anything unless he sells" as well. Last time I checked, Forbes assesses a person's VALUE by the value of what they hold.
Also, I am not sure I agree with your assessment of GE. There was a time that folks said the same thing about GM and AIG and Bear Stearns and Enron, etc....
Although not directly correlated, my experience has shown that when property values drop, my vacancy rate goes up. Two years ago, the 10 properties in the Fresno area were fully rented, but today, six are vacant. So we are trying to rent them, and quite a bit less than just two years ago. Therefore, when property values drop, so does rental income.
There is the theoretical side and then there is the reality side. CASH FLOW IS KING. Every formula, every 'abbreviated' investment term, EVERYTHING, hinges on the CASH FLOW. When cash flow is zero, none of the formula's work.
But to get back to the original question, diversity is a good thing.
As a side note, I took over my parent's properties because dad is in the advanced stages of Alzheimer's. That is one WICKED disease.