Nintendo's install base shrunk from 250 million in Gen 7 to ~80 million in Gen 8

[Cloudman]

What more can be said at this point? The decline is not exclusive to Nintendo (Sony also dropped in sales, the Vita especially from the PSP) and you're comparing Nintendo's best gen with its worst. As for how the Switch will do, it's hard to tell, but so far it's doing well. Also, because it doesn't have the success of something like the DS/Wii line, doesn't mean it's a failure.