One thing that many folks don’t understand about market saturation is that these devices are for individuals, particularly the handheld/hybrid devices. Every year more and more children become of age to have a console. The Switch’s lifespan has been so long that children born on launch day are just now old enough to have their own system.
Also, the anti-Nintendo bias is a strong thing (just like with any company) and sometimes it takes several years for that to wear off (if at all, of course). I’m sure there are many PlayStation and XBox fans who were very resistant to the console but only in the last few years have decided to take the plunge.
Market saturation is certainly a thing, but so is market growth. Granted, the saturation is far, far more powerful than the growth at this point, but the console still has years to limp along with modest sales figures based on variables like these.