I think it's more that the NS' hybrid form factor puts it in two markets and it has one market mainly to itself and the third party output retains value in this particular market even if a port is late and has compromises giving third parties more success than they expect as people are willing to get the NS version or even double dip due to portability.
Nintendo fans have had ports to choose from in the past but have declined. This generation has been different, and that's because Nintendo fans are buying these games, and they're buying them because they're getting solid ratings from reviewers and Digital Foundry. I'm not going to say that your reasoning is flawed, because it's obviously not and probably weighs the heaviest, but Nintendo fans don't gobble up every third party game just because it's "popular" or was the thing to buy last generation. They're picky and discerning and the developers are aware of this.