The Wii was cut short early when Nintendo effectively abandoned it after 2010 when development was moved to 3DS and Wii U. There's no reason for them to do the same with Switch as they no longer have to worry about supporting two different product lines. As such, the Switch will likely have significantly better legs than the Wii beyond Year 4.
Exactly. Nintendo basically ended its support for Wii when sales dipped below 20 million a year. Looking back the Wii was actually one of the least front-loaded Nintendo consoles with sales holding rather steady, and had it received proper support in 2011-2012 I could easily see it selling around 120 million units lifetime.
It certainly looks like Nintendo will not repeat the mistakes it made at the end of the Wii's life with the Switch. It won't reach the Wii's highs, but it will be around longer.