I'm not sure that I'd trust any of the exchanges to cease operating and return all user funds/BTC if their businesses are not longer viable due to falling revenues. I doubt that any of them have the kind of significant reserves which would allow them to "tough it out" for an extended period - they're all fairly new and the period when high volume and high price was giving them good revenue was fairly short.
Well, we're going to find out. The volume in currency on Mt. Gox is declining steadily. It's about half what it was in August. The amount of cash in the system is dropping as the price of Bitcoins drop. (You might think that as the price of Bitcoins drops, the cash would remain the same and more Bitcoins would be traded, but that isn't what's happening. There's probably even less money in the market than there appears, since the 'bot traders are mostly trading the same funds back and forth.)
It's not the exchange's money. They're just custodians of it. Mt. Gox is at best a money transfer firm under Japanese law, and money transfer firms in Japan are required to keep customer funds separate from their own funds. If they haven't done that,
it's theft. Unlike a regulated bank, a money transfer firm in Japan is not allowed to do fractional reserve banking.
We know who runs Mt. Gox.: Mark Karpelès. If it comes to that, we know who goes to jail.
Meanwhile, get your cash out of Mt. Gox. If they're honest and have all customer cash and Bitcoins, it won't hurt them. If they're not, better to know sooner than later.