Well, Nintendo didn't have much of a choice. The Wii peaked in 2008 in the U.S. and Europe, and in 2010 sales declined from 2008 levels by 30% in the U.S. and 33% in Europe. In Japan, it peaked in 2007 and by 2009 its sales had dropped by nearly half. If they put more effort into supporting the Wii with big first-party titles, they may have staunched the bleed-out a little bit, but it would be delaying the inevitable by maybe a year, and Nintendo would still be feeling the pressure to replace the Wii with a new system. A system usually has dropped 40-50% from peak year sales the year before they're replaced. The Wii's sales trajectory was, when adjusted for the size of the system's install base, rather normal for a Nintendo console. In fact, the Wii lasted longer than the SNES, N64, and GC did before being replaced. But Nintendo was running out of people to sell the Wii to.
TL;DR, the Wii wasn't cut any shorter than any other Nintendo system. It had a rather normal life cycle for a post-SNES Nintendo console.
In any case, the Switch should do very well. 100 million is certainly possible, though far, far from guaranteed. I think 80M is a more reasonable estimate. The ultimate test for the Switch's sales potential in the long run will be if and when it becomes the home for main series Pokemon games. Assuming historical patterns hold, we'll probably see the first "Gen VIII" games debut exclusives on the Switch, with a new main series game followed by an enhanced remake of Diamond & Pearl sometime later. Those games will almost certaintly propel the Switch to new heights, but how high those heights are will give us a clue as to what the Switch will likely sell lifetime. Other important tests for the Switch will be whatever updated models are released, the effects of price cuts, and perhaps also other major software releases, most notably Super Mario, Mario Kart 9 (if we get one), and Smash 5.
Nintendo would have benefited a lot more in supporting the Wii rather than releasing the Wii U.
The bolded part is another reason to think the Switch can go beyond 100M. The system is doing great numbers while the price is still at it's "highest". As for the software, it is clear that Nintendo is making an effort in pushing quality software this generation. The release of Breath of the Wild is the beggining, we can see they are on the right track.
"I've Underestimated the Horse Power from Mario Kart 8, I'll Never Doubt the WiiU's Engine Again"