I believe closing MtGox would crash the market for only for like 1 day... The real crash is when government announce that Bitcoin is illegal...
And how would they enforce this? The whole point of bitcoin is anonymity and protection of identification through addresses. Why do you think it took them so long to find the owner of silk road? And yeah silk road 2.0 is up now allegedly run by the same guy. So good luck government banning bitcoin it would affect the price, but also bitcoin is a global currency. One government outlawing it wouldn't crash the price imo.