Take a look at volume in MTGOX. In usd it is something around 200 mil a month. The rest of exchanges are far away. Why MTGOX shall fail?
The only thing that is bad with them is the withdrawal time.
That mtgox volume you speak of is the sound of a large number of people exiting gox any way they can. Back in Jan of this year I did a withdralw from gox to my bank account in Aus via wire, it took under 24 hours. I have a similar size one at the moment - I sent it in late June, it still has not turned up. I would be very carefull if using gox to sell bitcoins.