On the plus side, you have to ask what's being done with the "old" units. When someone buys a new PS2 to replace their broken one, the old one goes in the trash (pretty much). When someone buys a DSL to replace their old DS, there's a good chance they'll trade in that old DS (meaning, in theory, that someone else is going to buy it and, more importantly, buy games for it) or sell/give it to a friend/family member.
The point is, repeat sales on the DS (usually) still leaves the original DS unit in the food chain. Repeat sales on the PS2, typically do not.