Only because noboby will talk about the switch then anymore
That's a terrible argument. Switch having a great 2017/2018 didnt prevent PS4 from having its 2 best years which shows that the 2 ecosystems can coexist very well without harming the others sales.
Also 3DS has sold like an additional 40 million after PS4/XBO released so there really is nothing that should cause anybody to think Switch will all of a sudden stop selling once PS5/XB4 release.
When the herd loses its way, the shepard must kill the bull that leads them astray.