I think you're arguing something that we've yet to see. It's easy to turn this into an emotional argument and say that 3rd parties don't support Nintendo because they "just don't want to," but frankly I can't say that Nintendo's honestly courted 3rd party support with observable effort, especially in the face of two strong competitors who have realized the value of 3rd party support and bend over backwards for it. At the end of the day, that's what it comes down to.
Also, we need to stop thinking Nintendo downplayed graphics parity with other next gen consoles because the result would have been a super expensive machine. That's not true. The fact is Nintendo could have ended up with a much more graphically capable machine, but that's not what they chose to prioritize with the hardware. What they chose to prioritize was low power consumption and a small form factor, and so they commissioned a "relatively" expensive customized chip to achieve these goals. Only problem is people don't really care about these things to the degree Nintendo expected.
The Wii U isn't as underpowered as it is because this is all Nintendo could do and still keep the price low. The Wii U is underpowered because, like pretty much everything else having to do with this console, Nintendo made a bad bet on what consumers would care about.