Do you know what the DS needed? It needed touch screen controls for GBA games. Because in the latest Pokemon games you don't need to use the face buttons to call up the menu and stuff like that, just the D-pad to move around. The DS should have had that functionality, which would have been cool to use in turn based RPGs and strategy games like Pokemon, Fire Emblem, Advanced Wars, Final Fantasy, Harvest Moon, and a few other titles that could be played with just a D-pad and a stylus.
 
It would have been great if Nintendo included a GBA slot for the DSi models. But alas it isn't, just a dream that won't come true...

Maybe it would've been nice, but there would be way too much work to integrate the touch screen into those old GBA games for it to be worth it.

GBA slot in the DSi wouldn't have been bad, but it's not really that much of an issue.  There are so many alternatives now to playing GBA games that, if you own and want to play your GBA games, you won't just have a DSi as a portable.
 
But adding a feature to an old game is absolutely ludicrous.  The amount of work that would take... the deals with developers/publishers to effectively modify their game for a new platform... it wouldn't work and, honestly, those games don't need it.  They still work fine without touch controls.