For the games I was mentioning it's not the 3D being essential -- they're DS games (except for the 999 sequel). But Hotel Dusk for example is played holding the DS like a book, and that wouldn't work at all on the 2DS. In at least one case that I remember you have to be able to open and close it to solve the puzzle.

I guess if someone still had a DS to play DS games they might not care though.