750k is not inconceivable. What is inconceivable is that a significant portion of those "coins" were highly liquid. I agree that if the numbers are that high, it is largely in part due to the use of Gox as a web wallet. The fact that Gox was an exchange is not evidence in and of itself that all coins held there were readily available on the market. Just take a look at the order books -- it puts things in perspective.
If only a smaller fraction of these coins were liquid (say, 100k), then the remaining 650k were for holding, presumably using Gox as a wallet.
I still can't imagine that 6% of all coins have been held at gox, at a time when everybody should have known for month what kind of company they are dealing with.
I am surprised people didn't know better too - but think about the number of coins kept at mining pools when for years there have been examples of pools having the wallets emptied or going bust due to bugs. And yet, miners still keep BTC at the pools! They should at least have an auto-payout at 0.1 BTC or use instant payout pools.
:-)
edit: e.g. bitclockers, slush (vps web host inside job they thought), Hashcows etc...it has been an ongoing problem for years.