Here is an
example of a peg failing. If the Bank of England is unsuccessful in enforcing a peg, what makes you think random actors can enforce one? All it takes is one dedicated individual with enough money or a systematic breakdown in confidence across the board.
The problem there was that the British pound was a British pound, and its value could fall in terms of other currencies it was supposed to peg to.
But a bitUSD isnt a federal reserve note dollar.
A bitUSD is "This is $1 worth of Bitshares, and the blockchain promises it".
If you want to make a bitUSD, you need to put up $3 worth of bitshares as collateral.
Then, the blockchain constantly checks the value of your bitshares collateral. If it gets down to $2, because BTS went down, the blockchain says: "You are getting a little bit close to not being able to fulfill your promise to give him $1 worth of Bitshares. We cannot allow you to get into a position where you might not be able to give him his $1 worth of Bitshares. I am now issuing you a margin call, you are now forced to give him $1 worth of Bitshares for his bitUSD".
When the Bank of England had the problem that the British Pound was falling, they hadn't set aside a bunch of Deutchmarks or whatever as collateral, and no blockchain was forcing them to remove some of the pounds from circulation and give everyone the pegged value of marks or whatever in return.