Some have said that such an event would have a cleansing effect, sweeping clean all that has been foul in the financial sector. Of course that foulness might reoccur afterwards, but likely not in our lifetimes. Who knows?
I think it's a legitimate argument. And some people argue that rescuing entities that are "too big to fail" creates the environment for more bad risk-taking (the so-called "moral hazard"). But there's also the argument that we were faced with another major depression which outweighs these concerns.
The Great Depression resulted in regulation to reduce the chances of it recurring. If there had been no regulation, I doubt there would have been a "cleansing effect," or at least one that would have lasted very long. People are greedy, and that won't change. So I'm skeptical that letting everything fail would have been to the long-term good.
But if we don't follow up this crisis with a retooling of our financial system, there's little reason to think that this event won't recur either.