There is a difference between backing and pegging.

Backing means that the holder is guaranteed something of hard value, which traditionally requires a promise by a central authority in the case of paper money.  The guarantee of hard value is only as good as the word of the central authority, which may or may not be good depending who that is.  Coins that contain metal themselves, such as silver dollars, are guaranteed value because if the value AS CURRENCY falls, then they still have the metal which they could melt down and use as bullion.  The holders of silver dollars do not need to depend on anyone's promise.  The backing is decentralized.

Bitcoins do not back themselves because they have no use other than currency.  You cannot use them except to spend them, and you cannot spend them without someone else accepting them.  Silver dollars and copper pennies have non-monetary value because you can melt them down and you have metal which has industrial uses.  Bitcoins do NOT have this type of value.

Pegging a currency means manipulating the market to maintain a certain exchange ratio.  Traditionally it's been governments who buy or sell currency to maintain the desired exchange ratio.  Governments LOSE MONEY when they peg a currency.  But they deem it worthwhile to maintain confidence and stability.  Because it's a money-losing proposition, market participants are not in a position to peg a currency.  Hoping that speculators will effectively peg a currency at a certain ratio is the same as hoping that the market in general will peg a currency.  Not going to happen.

Through mining, you can flood the market with new currency if its value rises above a certain level, but there is no mechanism to contract the quantity of money if its value falls.

Protection against rising value is not really protection (except for borrowers I suppose).  Falling values are really what you need to protect against, to give people confidence that their currency holdings won't be wiped out.