If people can rely on its value, then there is no reason for them to cash out into their bank account. In fact, there may be every reason in the world to move your money out of your bank account and into Steem Dollars.

You forget the people that don't trust dollars but prefer for example Euro, Yuan or even Gold on their "Bank accounts" even if the Steem Dollar peg works wonderful... Don't underestimate the anti-Dollar-FIAT trend ! We should have more options than SBDs on our platform! let the users choose what "stable" currency they want hold and they will never blame "you" if they fail...