Just because someone gives it a formal name doesn't mean it's true.
>One person's savings is another person's debt.
Only if you treat fiat currency (or bank deposits in general, the paradox you mention was formulated long before we got off the gold standard) as the only possible form of savings.
Sticking gold in a vault is only a means to saving if you have a reasonable expectation that when you take it out you can use it to obtain goods and services. It is thus a disguised claim on future production, and so relies on debt to exactly the same degree as bank money.
That single word is doing all of the heavy lifting for you. "If" the housing market will never crash, then it's a surefire safe investment! Better buy tons of houses to flip on credit. There's no guarantee that your debt/investment will succeed, which is why banks try (and often enough, fail) to price in risk with things like varying interest rates, collateral, etc.
the 90% accounts for the "if". my point is there an expected value for the future revenue amount, subject to your own assumptions about the probability of each outcome and your discount rate for the value of that money over time
in scenarios where your expected value discounted to present value is greater than the alternative, you make the investment. it's really finance 101... it's just NPV
