The one annoyance of the GBA at its release and original lifetime was the screen. It wasn't backlit and was very difficult to see without perfect bright lighting in your room. They eventually fixed this with the backlit GBA SP some years later, but gaming on the original GBA was rough. It's easy to forget just how bad things were without backlit screens, we've been spoiled by decades of amazing smartphone screens in the years since.
I assume most did not believe Nintendo's statement on the matter.
https://www.gamespot.com/articles/nintendo-ds-not-game-boy-a...
It didn’t really work and Nintendo pivoted—possibly also to compete with the PSP—but I see the Gameboy Micro as evidence of their strategy. Compare an original model DS and the Gameboy Micro side-by-side—the size difference isn’t entirely unlike that of an iPhone versus an iPad Mini.
It’s not the first time Nintendo tried to create a third category between console and handheld—see also the Virtual Boy.
The DS being able to play GBA games was a huge selling point, as people forget how "radical" the dual screens were and how uncertain it was that people would adapt, so having the GBA fallback was nice.
Also, the DS was a better GBA than the GBA.
IMO a backlit SP is the best stock GBA you can get.