As a parent of 3 relatively young children (ages 10 to 5)...one of whom has a 3DS, I can tell you right now where the majority of their casual gaming occurs: my "old" iPad 2. With the current 40+ games installed on there, they could each easily burn 1+ hours if allowed. In terms of everyone comparing touch vs "real" controls, I can tell you that this generation doesn't care and can easily jump between both without a second thought. If we're going to compare graphics, most modern mobile devices (both iOS and Android) blow away the 3DS hands down.
Now as a UI designer, the DS has a very unique interface when gaming that is pretty cool and is their signature hands down. That's not to say that it cannot be emulated on say an iPad or Kindle sized device to recreate the DS gaming experience. As another user here commented, release just one game in the App Store and Play Store and see what happens. Just look at the success that ActiVision has had with the Skylanders franchise on mobile devices. Oh, you mean I can leverage my investment on *2* separate gaming consoles?....HERE! TAKE MY MONEY!!
I don't believe that anyone here is suggesting that Nintendo completely dump their HW business (with the exception of the Wii U). What most are asking is why NOT release something for iOS and Android and grow your mobile business even MORE give the immensely strong catalog and gaming experience that Nintendo brings to the table. Let's face it, most people in this forum have played one or more Nintendo game and as such, most children of these folks will to. There comes a tipping point where a device such as the DS is no longer desired (think teens) with the exception of those man-children you see at Pokemon tournaments.
Mobile devices whether they be Android or iOS are only going to increase in adoption. It would be foolish for Nintendo to completely ignore this market space long term. The numbers alone of kids AND adults with these devices significantly outweighs the total DS market. I leave Nintendo to seriously ponder 2 specific words: Angry Birds
From nothing, Rovio created an empire. Just think of what Nintendo could do with their existing offerings.
-K