The numbers tell a different story. Last year, between April and December Nintendo sold more than half a million GBA games. That's pretty darn good for a system that's over eight years old at this point, and it's definitely enough to counter the claim that "people don't even play GBA games anymore." Oh, and that figure doesn't include sales of used games, which are reportedly increasing (as sales of new games fall due to the state of the economy).
Plus, that's only games that were BOUGHT last year. All of the games were bought the year before that (during which over nine million GBA games were sold) will probably continue to be played into 2009.
Realize that the economy in the United States (where most DS units are sold, if I'm not mistaken) is in pretty bad shape. Already the sales of used games have been increasing. Why? Because people are realizing that they don't need to pay $60 for a new game when they can get a used game for $35. What do you think people are going to do when they realize that used GBA games can be had for $10 or less in most cases?
But let's completely ignore that fact and consider that
the 19th best selling DS game in North America REQUIRES a GBA slot to play. It's not like Pokemon where the GBA slot can optionally be used to add gameplay elements. Either you have a GBA slot, or you don't play.