It's not. Six axis can't provide what gamepad can. And I don't exactly have to swivel my head to see the hud, all it takes is a quick glance. Touch pad can't do what touch screen can. You can't map out touch screen buttons on a touch pad. And I love full screen view without having to pause for the map, it's actually quite helpful that you can zoom in and out on the gamepad.

Persona is a great example of a good spin off.  Technically you could call Mario a spin-off of Donkey Kong, but really it was more like a sequel.  The villain in Mario's first game was Donkey Kong, then (after his villain turn for one episode) his brother was introduced and he started fighting turtles in the sewer.  The sewer lead to the mushroom kingdom and so on.  Each Mario game (starting with Donkey Kong) was an expansion and improvement on the previous one with the same hero and basic game mechanics.

Luigi's Mansion is a good example of a true spin-off.  And it's also a good example of why the idea presented by the indie in the OP is stupid.  If you had one system getting SM3DW and another getting Luigi's Mansion 3, almost everyone would buy the SM3DW system and none the LM3 system.  That's what has happened with Nintendo's systems as of late. While PS360 gets FFXIII Wii gets FF:CC.  3DS gets China Town wars instead of a true GTA like the PSP.

A major problem Wii U has is that after the Wii in 7th Gen, people don't view it as a platform that will get AAA 3rd party titles.  As a result they aren't even buying it, and spin-off titles won't change that.  People want a system that plays the main series games.