Isn't this totally false? He could have easily made backups of the wallet and even given copies of it to others. I'd expect there's a whole bunch of ways that these bitcoins could still get spent?
(Of course, there's no way this could happen since we don't know what private keys are stored in Ross' head. Also this is based on the efficient market hypothesis and real world results have other complecting factors.)
The other question is then whether the FBI would then liquidate the coins via an exchange, or just sit on them.
Not true; anyone who sells something for bitcoins creates some amount of intrinsic demand. Silk Road did, and was probably by far the biggest generator of such demand, but not the only one.
People think dollars have value because they're accepted by other people because other people think they have value.
You accept your paycheck in whatever currency you accept it in and believe it has value because you have faith that other people will accept that currency for whatever goods and services you want. It all works as long as everyone believes the currency will keep having value in the future. Bitcoins are no different. As long as a group of people believe they have value in the future, and will be accepted as payment for goods and services, they will continue to have value.
If he stored the bitcoins in a single address, and he has the private key stashed somewhere, then even if the wallet file is deleted, it can be retrieved. This is how bitcoin works, if you have the ECDSA private key of a bitcoin address, you can spend those bitcoins without having the need for a wallet :)