It always surprises me how people act like 3rd party sales are completely unrelated to 1st party sales.
-reasons-
I agree, but not for the reasons you gave. A console will sell based on the quality + quantity of content that its audience wants to play. The problem with third-party sales on Nintendo platforms is that many of the games third parties have released on Nintendo platforms (particularly since the DS and Wii successes) have fallen in one of the following categories:
1. Games that do not align in any way with first-party offerings
If you watched the rise of the FPS, Sony's dominance of JRPGs, and other industry trends, you might have noticed that the types of games that dominate a platform are the types of games that the first-party platform maker is investing in heavily. This applies to Nintendo platforms, too. So why would the kinds of games that Sony/Microsoft dominate be a good fit for Nintendo consoles in the first place?
On the flip side, Nintendo could definitely do more to invest in the genres they've historically neglected. So it's not on the shoulders of third parties entirely - they just have shown poor judgment in the titles they release on Nintendo consoles. A good example: Square-Enix releases a Deus Ex remake on Wii U, but not the family-friendly Disney-themed Kingdom Hearts 1.5?
And, no, Nintendo, you're not off the hook for RPGs just because you're making Xenoblade. You need a variety of content for your audience for it to be viable for them or for third parties, not just one game.
2. Low-effort, low-quality games for "casual gamers"
This one should speak for itself. Wii Sports was obviously not shovelware; it was a major, strategic title for Nintendo and was the face of their console's brand. Even Kinect Sports/Adventures did well when they were given the appropriate level of effort. But you didn't see this level of effort from most "casual games" on Wii/U. That audience is culturally beneath most of the game industry.