I think we could survive a freddie or sallie hiccup. They are much smaller than fannie. If fannie implodes, it could be the end of the world as we know it, depending on exactly how bad a shape fannie is in.
The pure derivatives damage from a Freddie crash would not sink the economy by itself, you're exactly right.
For some reason, though, you seem to omit from your view the very same 'fear contagion' factor that has infamously pushed even a relatively over-supplied WTI mkt above $70/bbl.
(referring to physical futures mkts)...''Bull markets result from fear. Wildly overdone bull markets feed on themselves and the fear of the market participants that they might not be able to acquire the goods they need in future. When fear rules markets, there is no top end.'' -- SAJ, in Trading Options To Win, John Wiley & Sons, 2003.