The scenario with a large number of people wanting to cash out already assumes Bitcoin lost as a currency, so I'd say it could be a result, not the cause of Bitcoin's failure. Bitcoin succeeds or fails on whether you can buy useful stuff with it, buying other currencies is secondary.
The scenario with people cashing out because of regulation also assumes that most bitcoin users live under a single regulatory area, which is untrue. Banning bitcoin in the United States, for example, might decrease it's value, but not destroy it, as there's the rest of the world.
Not to disagree about the main point but US could do more to harm Bitcoin than just banning it internally, especially given that China seems already hostile to Bitcoin.
A bank run would be as devastating on any other currency as it would be to Bitcoin of course. Loss of confidence in a currency destroys it. The same is true for the Yen or the Euro or USD too.
