Of course not, but if a large percentage of people had both, then you wouldn't be surprised to see Switch sales being around half of what the numbers were before. I imagine Nintendo generally lost many consumers during the WiiU era, where they were confused about how the WiiU fit into things, and I also imagine a lot of people who liked the 3DS for being ultra portable might not be that interested in a Switch that is way bigger.You can't just count every handheld user as someone who also had a console.
And it isn't just about the handheld format. The argument is also about the specific genres that console/handheld gaming has either abandoned or marginalized. Many of those customers don't just shrug their shoulders and eat the oatmeal that is left on consoles. They either abandon the hobby, go mobile, or go PC.
But I disagree that Switch numbers mean that they lost half their consumer base. I think reality is they kept somwhere around 70 to 80% of them.