If you'd never played 3D land, you'd never think the visuals in 3D world resembled a handheld title, at all. 3D world was fine, despite not being what 'hardcore 3D mario fans' wanted. To the average consumer, it'd be indistinguishable between a 'traditional' 3D mario, I'm sure. I don't buy this argument at all. I just don't think Wii U gave your average consumer a need to buy one, and the best mario game in the universe wouldn't have changed this.
The point would be to use Mario to promote some game that at first glance obviously couldn't be done on the Wii or 3ds. To the "average consumer", 3d World was just a nothing that didn't attract attention at all. In that sense, their reactions weren't different from the disappointed hardcore fans who complained about the game resembling a portable game during its unveil.
Now, one game by itself would never "save the Wii U", but it easily could be a flag that attracted more attention to the console and Nintendo's overall output, rather than something that was revealed and released with little reaction aside from the audience that was already there.
NX gen should have one focus, regaining/reestablishing a stronger userbase of what you could call Nintendo fans. They won't make masses of gamers move consoles, that's ridiculous. It's also reaching to say that the audience of NX won't be interested in western franchises like ... CoD or whatever. The big annual IPs of the western 3rds have that special mainstream appeal, that's why Nintendo needs them on the system. And even among "Nintendo fans" those game have appeal.
They "have their appeal", but is that appeal really big enough to justify further porting costs? The NX will be coming in the middle of the generation. There will be little reason to get a console for games that are available elsewhere, so at best you'd get people with cross interests, who love Nintendo's games enough to get the hardware, and also like some 3rd party games, but not enough to buy other systems for them. That would mean generally low 3rd party sales. And then, there's also the spring launch, after the holiday season, so most fans of the games getting ported already would have them.
Really, the best 3rd party support that one should expect for the NX should be closer to the 3ds' than to what the HD consoles get, and that's a best case scenario. It easily could end up just as badly as the Wii U depending on the design decisions regarding the future console and handheld, alongside their priorities.
What if the NX handheld costed the same as the console, even if despite having weaker innards they'll still have to be in a relative same ballpark and the handheld has to account for more costs like screens and the sort, while consoles can offset some of the cost via accessories like controllers and the like. Similar to how the Vita TV was a lot cheaper than the handheld Vita (despite being heavily gimped)
I think that they'd avoid a high price for future handhelds due to the initial failure of the 3ds. Children are still a really important market for handhelds, so they need to be cheap enough to be gifts. Also, even in a scenario where both console and handheld are effectively a single system, I'd expect the console to have boosted inwards compared to the handheld in order to allow extra OS features, graphical enhancements and multiplayer in a single device.
Regarding their delay, I'd guess it's mostly a matter of focusing on their internal development. Their launch titles wouldn't be ready for December, but could be finished with some extra months. Alternatively, like mentioned above, there's the chance that they just want to get one final year out of the 3ds, and that's the actual reason for the silence regarding the NX, rather than it being a sign that any 3ds replacement is something for the distant future. Maybe they're waiting for the Pokemon launch even. Either way, the spring launch makes no sense if you attempt to look at it from a 3rd party support point of view.