According to Mat Piscatella "Next Gen consoles sales will not have a major effect on Switch sales"

[VideoGameAccountant]

The PS3 and 360 didn't have any affect on the sales of the DS. Same for the XBox One and PS4 for the 3DS. Handhelds are in a different market and don't directly affect others. So naturally it's not going to have a significant impact on the Switch which will sell for it's games and portability which the other two can't deliver on.