Regarding the GameCube, it also suffered from a lack of third party titles and people knew this (where was GTA?). Just because the third party situation was better than the wii u doesn't mean it was good third party support (again, the wii u has virtually zero third party support). Even then, the GameCube still out sold the wii u.
Likewise, your entire argument also looks at only the Nintendo line. I find it funny how you are looking towards the most unsuccessful consoles and having them represent all of gaming. Let's take a look at the most successful console in history, the PS2. That chart is dominated by mainly third party titles (and no gimmick to skew these numbers either). Third party titles made it into a giant behemoth. Same story with the PS4.
Third party matters most. Nintendo ignored this and for that there are hundreds of games that are now exclusively NOT on the wii u causing it to suffer. No amount of first party games can recover from that
Third parties dominate being they are available to TONS more people than Nintendo 1st party, and it doesn't stop Galaxy, NSMB Wii, Mario Kart Wii, ect from being some of the top games ever sold.
Let's completely forget about bundles because those don't skewer numbers whatsoever! /s
Likewise, the success (and early downfall) of the wii can be attributed to third party titles. Whereas early on we saw the huge success of guitar hero and just dance, later we saw the rise of call of duty, skyrim, battlefield, and other hardcore titles. This is what led to the PS3's comeback and what solidified the success of the ps4 and Xbox one over the wii u.
I find it funny that you are literally only looking at Nintendo consoles for evidence (the majority of which lose in their respective generation). Just look at any Playstation or Xbox console and you see that third party matters most