It is very possible that it won't matter one iota that the prime mortgages are performing much better than the subprime that is getting so much press. As the Alt-A and prime mortgage MBS get revalued downward, even in smaller amounts than subprime MBSs, the holders of the MBSs have to throw them on the market at a less than profitable trade because the MBSs have been used as collateral in leveraged trades. As the MBSs get revalued, the holders get margin calls. As the MBSs go on the market and have to be sold at whatever price the seller can get, all similar MBSs get revalued at the new and latest selling price. It creates a downward spiral, exactly the opposite of the upward spiral that created all the loose and expanding credit. Goldman Sachs can not go out and use a pile of MBSs as collateral for a loan if it is unprofitable to do so. And they are hesitant to purchase more of the same thing. Now, do you see why it may not matter one iota how good the underlying asset is?<p>
And none of this is even taking into account all the credit default swaps that the big five wrote and may now have to make good on. If they start paying out on CDSs, an MBS for sale will look worse than ... , well I can't think of a good analogy right now.