<em>"What if all the potential buyers already bought?"</em>
IMO, this is what started the slowdown. When a bubble bursts, volume drops dramatically due to buyer fatigue.
If more credit became available, it might slow the decline as it would increase sales. Remember we saw continued appreciation early this year at the high-end even while the low end had no sales mostly because prime borrowers could still obtain credit. Particularly here in California, if someone is willing to loan money, there will be someone willing to borrow it. Are there enough to these potential borrowers left to save the market? I don't think so. If there were, sales would not have dropped off from late 2005 through the present.