Although we cannot know for sure at this point if a change in the party system has occurred, I would argue that the subprime mortgage crisis did not cause any such change.
First, I will argue that there was not a change in the party system. It is true that Democrats won the two presidential elections after the crisis and that Republicans had won the two before that. But before President Bush’s two terms, President Clinton, a Democrat, had won two terms. So it is not at all clear that President Obama’s two elections constitute a change to Democratic dominance.
Even if there has been a party system change, I will argue that it was not caused by the mortgage crisis. It is possible that Obama’s election was brought about largely by that crisis. Even so, that was just one election. If the Democrats are to dominate from here on, it is going to be because they dominate the rapidly-growing Hispanic vote and because they get the votes of the many younger voters who are turned off by the Republicans’ attitudes on gay marriage.
Thus, I would argue in at least two ways that the crisis did not lead to a party system change.