@Dravenae
Well Gamecube and GBA games are old and not sold anymore in mainstream stores, like NES, GB, SNES, GBC, and N64. DS games are still being produced, Nintendo is still making copies of most of their first party titles. Downloadable versions can possibly be released eventually, but it does not seem likely any time soon.
It could maybe take a little away from 3DS sales, but there is still a point to buying a 3DS, portability. The attachment would probably cost almost as much as a 3DS anyway, judging by the Game Boy Player, it would probably be a 3DS without screens or a battery that plugs into your Wii U. So any lost 3DS sale would be made up by this being sold, plus it give a reason for people with a 3DS to buy one, or a reason for people without a Wii U but a large DS collection to buy a Wii U, like me. And Nintendo would still make money in 3DS software sales.
3D would probably have to be cut from the Wii U one though, 3DTVs kinda died.