GameBoy Advance isn't dead yet.  Personally.

It's a good platform, it's convenient, and it still offers good games.  It isn't dead just because the value is somewhat depreciating this year.  Just because there are lot new consoles, new platforms, and better games not meant for GameBoy Advance out there to haul it down.

I own a GBA SP and I still play with it.  Besides PSP, GBA is a very convenient piece of entertainment stuff.  You can carry it with you wherever you go and play with it anytime and anywhere with anyone (for as long as the battery power isn't out).  Plus, GBA games are sold very cheap so many people continue to buy them.  And so GBA isn't dead yet.  Just like Windows XP, it's still alive and "kicking".