If you look at this and think I'm making a stretch, I don't know what to tell you.
The Wii is an anomaly to a long term trend of decline that has persisted over decades.
This is factual. Not sure what he's talking about but he's going to be wrong. Every single Nintendo console has sold less and less than the next. There's a factual reason or this. Factually, Nintendo's fanbase has left every generation after not dealing with their systems, lack of games, or just in general, policies. Every generation they lose more and fans. What do you think makes up the bulk of PlayStation and Xbox's fanbase? Ex Nintendo fans.
The Wii was a fluke. A product of its time. No matter how much you think it will...it will never happen again. The age of digital, internet, and iPhones was in its infancy. Wii hit its stride due to the year and times it was in. If you think Switch is going to sell as good as Wii or even close to it, you need a reality check.
No one should argue this because that's legitimately what it was. After the casuals gave up on it it was a fucking wasteland. Commercially? Yes, Wii was a success? For gamers? Wii was a failure. It lacked games aside the usual Nintendo 1st Party and had absolutely 0 3rd Party games PS3/360 had. I remember when SS came out and how dead in water the system was during that year. It all went downhill from 2009 onward. The casuals left and hardcore stopped caring cause the fad was over.
A great example to compare it to is Pokemon GO. Except that fad didn't even last 3 months. At least Wii lasted 3 years before people moved on.