And if MtGox gets hacked? Or targeted by law?
I'm not going to answer the "hacked" part of your question (mtgox was hacked already), but the "targeted by law".
Bitcoin is currently legal in the countries that protect free speech and trade.
Changing the law in these countries is a long and complicated process, that tends to spark public debates.
And if you want to stop Bitcoin, a public debate is the last thing you want.
If they want to stop Bitcoin, banks and governments are more likely to use existing anti-money laundering laws.
What they can do with these laws is limited. They can target exchanges that rely on bank accounts, such as MtGox.
However, there is nothing they can do against semi-centralized exchanges such as bitcoin.de or bitcoin-otc.
Targeting MtGox with anti-money laundering laws will have zero impact as MtGox already follows anti-money laundering laws.