Well that problem will not exist with WiiU and so few DDR games are released on Nintendo platforms I don't think Nintendo sees it as a loss for Wii either. I mean the majority of Wii units can use the DDR mat's perfectly this new cheaper model is to please all those penny pinchers who didn't buy a Wii at full price. Its also meant to bring in every dime of extra profit Nintendo can milk. I mean with hardware sales dropping and Nintendo losing money on 3DS they need to claw in every dime they can. This means making the hardware cheaper which is easily done by removing GameCube functionality.
Lets face it DSi didn't have GBA functionality and it didn't suffer much at all. If a consumer wants to play GCN games or DDR that badly they can always locate a cheap used Wii or find a retailer who still has some of the old Wii in stock. Yah you can't find the old Wii on shelves in almost any major retailer but try a smaller game specialty store or online and I am sure you could find an old model without much of a problem!