Historically, collapsing banks simply collapse all at once. There is no incentive to slowly unveil a fraud. The depositors might kill you.
https://bitcointalk.org/index.php?topic=50822.0
So many investors held out much longer that many might have expected, clinging to the belief that pirateat40 was true to his word, and that the Ponzi stench existed only in the minds of cynical observers.
In the end though, the last groups of investors lost almost everything.
The regulators will also keep track of their reserves, particularly if there seems to be a problem.
None of this applies to MtGox; it's an unregulated company running in a country with very different law to that of the countries most of its customers live in.