Well that is an unproven assumption. MtGox has provided no clarity. We have no idea how many coins were stolen. MtGox has never explicitly articulated how many coins were stolen, by how many people and over how long of a time period.
As for "controlling" the exchange rate. So they dump it, ok now they don't have the coins anymore and the exchange rate will recover. Bitcoin rewards the "smart" thing to do. If an attacker had 800K BTC they could sell it off slowly and be rich for the rest of their lives, or they could dump it all, very likely never be able to cash it out due to AML/KYC type requirements, and cause a short term drop in the exchange rate. Are you always afraid of every shadow? There is no evidence that a single attacker has all the coins, and if they do there is no evidence they would be downright idoitic with them.
I mean do you really believe that Bitcoin can be completely annihilated by a single entity acquiring 800,000 BTC and selling them? If so why are you here? If Bitcoin can be destroyed by one person selling too many of them too quickly then it was worthless from the first day. Even if no thief current has that many, at current exchange rates buying a million coins wouldn't be beyond the abilities of a well funded malicious entity. It would be a tiny fraction of the cost of a 51% attack and your claim is that short of a network killing fork there is nothing that can stop them from destroying Bitcoin by just clicking the sell button.
Some of us believe Bitcoin will do just fine, and the biggest risk is rushing to implementing some dubious "fixes" based on little more than "we have to do something".