No the opposite would have happened.
As the coins would have come from wallets known to be in their control. Everyone would know that they were dumping the coins to Cause a price crash.
In fact whenever they choose to sell, it will just be a bargain for anyone that has money on an exchange and I think the move would retrace in less than a day.
There are more attacks than just the 50%+1 attack. Orphaning blocks for instance. Commandeering major mining operations, a virus (like stutnex attacking nuclear facilities), DDOS (NSA has a lot of influence over the main DNS providers.), them figuring out the identity of Satoshi (What is the NSA's job again?) forcing Satoshi to give up his wallet then we'd have the DPR problem but on an even larger scale.
This would be more than "panic selling" where even the loyalist to bitcoin (such as myself) would have to logically realize that getting out of the market would be the smartest thing to do.
As it is right now Bitcoin could not survive ANY of these attacks and so we are relying on the charity of the very organizations whom want to see us go away.
That does not sound like a very good idea.
The solution to this problem is a technological one that makes such a coin seizure unlikely or infeasible in the first place, not banning coins. The Bitcoin community will just have to accept the 144,000 already taken as motivation to do better.
I'm not suggesting a "Colored Coin Solution" but to alter the block chain so the FBI's coins no longer exist. I'm not sure if this is even possible? But something to consider.