They Wii could have sold so much more... The only good thing about the Wii U is that the Switch was made afterwards, you could say that they used the gamepad for inspiration to develop the switch.
I don't really see how it could have sold a lot more than it did. It didn't have the third-party support it needed to have better legs. Nintendo diverting resources away from the Wii U (which was in development since 2008 and was more or less in its final stage before E3 2011) to produce a few more first-party games probably wouldn't have helped, either. Had the Wii been a powerful console on par with the 360, it would have cost a lot more to manufacture and thus would have a higher retail price, which could have hurt its sales potential especially with more "casual" players. Having the same level of third-party support might have helped compensate for that, but then again the Wii would be competing directly against the 360 & PS3. It would have been entirely up to motion controls to sell the system and make it appear to be a better value proposition than the 360 & PS3. A $350-400 Wii with good specs and solid third-party support (but with the same heavy focus on motion controls) may or may not have sold as well as the Wii we actually got. There's no way of knowing since we can rewind history and change the circumstances. But the Wii that we got sold what it did, and it sold very well, but I don't see any scenario where Nintendo could have feasibly increased its sales by any substantial quantity, at least not enough to justify delaying and diverting resources away from the Wii U.
They're not, but they're relevant to Nintendo as a whole. Historically, Nintendo platforms sell less than their previous generations (with the anomalous exception of Gen 6), so it's not outside of the realm of reasonable predictions to believe it won't hit 80m or more.
That's not quite true. Even ignoring the Wii and DS, global sales don't tell us the whole picture, and don't establish an actual trend of any kind. Looking at regional trends, the biggest cause of the SNES doing worse than the NES was the Genesis being significant competition in the U.S., with the SNES selling at least 30% fewer units than the NES. In Japan, the drop was only 11.3%, because the Genesis, while it did better than the Master System, wasn't significant competition, though the PC Engine (TurboGrafx-16 to us Westerners) did a bit better. In Europe Nintendo was essentially flat gen-over-gen.
The N64 actually sold as well as the SNES if not better in the U.S. In terms of market share it did worse, but in absolute numbers it did just as well (in fact, it did better in its prime years). Japan was the main cause of the gen-over-gen decline in Gen 5, with the N64 selling a whopping 69% fewer units than the SNES did in Nintendo's home country. In Europe, the drop was a much smaller 22%, but Europe was still a very small slice of Nintendo's pie for home consoles. The N64's relative lack of third-party support compared to both its predecessor and primary competition hurt it big time in Japan, but in the U.S. the N64 was for a while the fastest-selling home console ever at the time, and its lifetime sales by the end of its first full year were surpassed only by the PS2, Wii, and PS4. It wasn't until FFVII came out that the PS1 pulled ahead, and even then the N64 was selling faster than the SNES was during their first four full calendar years in the U.S.
The GC was universally a big drop. It was down a good bit vs. the N64 in every major region: 35% in the U.S., 27% in Japan, and 30% in Europe. But Xbox was a big competitor in the U.S. (and UK), plus the GC's quirky design and relative lack of third-party support hampered it.
The Wii U was a big drop vs the Wii for reasons we all ought to be familiar with. The touchscreen controller was simply not the system-selling gimmick the Wiimote was. For one, it caused the Wii U to be more expensive than the Wii. Also it was not as easy of a sell, an issue compounded by the atrocious marketing for the system. Also, the 8-month software drough between launch and the release of Pikmin 3 didn't do it any favors.
For their handhelds, the GBA had the disadvantage of having a much shorter lifespan than the Game Boy and its 14-year run. But even then, in North America the GBA actually came dangerously close to surpassing the Game Boy (combining the original, Pocket, and Color versions like Nintendo does). It might have succeeded in doing so had the DS been delayed by a year. And it accomplished this task in less than 7 years. It sold at such a fast rate in the U.S. that by the end of 2004 (just over 3-½ years after launch) it had already sold nearly 27 million units. By comparison, the PS2 sold only 23.5M in its first 43 months while the Wii sold a bit over 29M. During its relatively short life, the GBA was an absolute monster in the U.S.
The GBA's beastly performance in the U.S. is why the 3DS has yet to pass (and probably won't pass) the GBA in lifetime global sales. In Japan, the 3DS passed the GBA a long time ago. In Europe the 3DS is still behind the GBA, but it could still catch up (though the 3DS will still have sold at a slower rate than the GBA).
When dissecting Nintendo hardware sales on a region-by-region basis and looking at the probable reasons why they sold what they did in each region, the idea that Nintendo's home console or handheld systems are on some long-term trend of decline, interrupted only by the anomalously insane performance of the DS and Wii, isn't clear-cut. In fact, whatever pattern there is may be purely illusory.