Well, cross-platform is much more common nowadays than in 2011, and Nintendo was pretty clear in pursuing the ecosystem approach Sony had been implementing since a few years. Also, on which platform MH3G should have been appeared on top of 3DS?
The Wii, probably? It had the original Tri, that sold more than one million, and even if it was already dead, Nintendo was still releasing a certain Zelda on the same november.
I don't know, I'm just saying that it already happened to see an expansion as exclusive release on a new "leading" platform. Of course it's more interesting for the platform older to see big releases for the new platform, but I also think that for major games there is a shared interest in pushing a new hardware.
We are seeing this from SE on PS4, for example, and in the portable segment (if NX will be portable), I'd say that it would be interest for third parties owners of very big portable Japanese experiences to push the NX, if it will really be the only dedicated console device in that segment.
I'd also say that it would be interesting to see Pokemon Z in the very same situation, even more if we think that, apparently, Nintendo's part in Pokemon should be slighlty bigger than in Monster Hunter.
But please note, I agree that it probably would not happen and both games will be 3DS/NX cross platform at best.