Anyway, this guy looks at it wrong. It isn't that demos make games sell worse (which could be true), but games guaranteed to sell well don't make demos (which is 100% true.)
Which best selling games have demos? I can barely think of any. They don't need them. They don't sell well because they don't make them, they don't make them because they know they'll sell well, often because they have a 4 or a 6 or an 18 in their name.