Whether Switch has a shot at dethroning the DS will come down pretty much solely to how long it's supported for. If it's not replaced til 2024 (and is properly supported til then) it may very well come out on top.
DS will be tough, to be honest I'm not sure if Switch can outsell it.
This holiday is a good test. Can it close the gap to under 2 million again? If yes, it has a decent chance.