I know personal experience doesn't always count for much but I encounter loads of West Ham fans. They'd have plenty of demand for tickets if they reached the Final, for example. I also honestly believe Millwall would sell out that many tickets at £20/£25 a pop if they had us in a Cup game and capacity allowed for it, and they have far fewer supporters.
I suppose we could give them the benefit of the doubt and appreciate they may want to avoid the Olympic Stadium at the moment...