You can bet that without 2DS, sales would be much worse than they are.
That can't truely be proven one way or the other without knowing how much of the overall sales are from 2DS, though I don't know how you can get much worse than a nearly 50% drop YoY. The 3DS XL atleast helped keep sales stable. However I think the new 2DS XL is another sign the 2DS didn't meet expectations. Just like 2DS is an acknowledgement that 3D is undesirable, so too is the 2DS XL an acknowledgement that no clam-shell is no good. It's truely the definitive 3DS model, if only they had launched with it.