DSiXL was just an offspring of the DS. The 3DS is Nintendo's next gen portable console.
So you're saying that the next-gen portable console cannot be a DS?
Why wouldn't they keep 'DS' in the name?
Of course, they did not call the original DS another Gameboy, but that is because it looks significantly different(X/Y buttons, dual screens, etc.). But '3DS' makes sense, because it combines the terms '3D' and 'DS' into one term, and though it has the 3D effect, the 3DS' shape and button layout does not look significantly different than the original DS. It still retains dual screens, X/Y buttons, and a stylus. Therefore, the 3DS could be considered an offspring of the DS.
If Nintendo were to make another handheld that was not a DS, it should probably look significantly different, in my opinion, with a completely different method of playing it.