The first few generations of DS were fully backwards compatible with the GBA though so the transition wasn't that bad. The Gameboy in general had an amazing run of backwards compatibility through its lifetime. The GBA could play original Gameboy games going back all the way to 1989.

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.