The lighted screen is THE biggest complaint about the Game Boy Advance, and that was solved. Adding a rechargeable battery and the slick new design was icing on the cake. Yes, this will probably end up attracting older gamers to the GBA, and I'm sure that was a large part of the intent. But Nintendo is just addressing a major concern of their userbase, the one thing keeping it from gaining major traction across all markets.
And an SP is NOT way more expensive than an Afterburner, if you're buying both outright. In fact, you can trade in your GBA at GameStop for $40, which is your pre-order payment for the SP, leaving you with a cost of $60 ... the same price it would cost you for an Afterburner and installation for your existing GBA. There's really little reason people shouldn't upgrade to an SP.