I rather find it sad that some people can't allow Nintendo to be succesful and try to discredit them in any possible way just to make themselves feel better. It's intolerance like this that I have a problem with, so I want to point out how flawed it is. The mission is to stop the hate and let every company in this business be successful so that we as a gaming comunity can strife and enjoy our common hobby.
Except EA, they're cancer.
But you're not even countering the argument being made - you're just taking it so literally as to try to create a substantive counter-point. The overall point that Nintendo can rely on it's core audience to buy it's software isn't even wrong, and it's not something that has to be negative either. Like many other things it's just a fact in a void, and how you interpret it or use it is up to you. That's why Wii U games sold so well despite the proportionally small userbase, that's why 3DS games sold as well as DS games despite the platform having half the installbase, because Nintendo can rely on their core userbase to buy their software. And what's wrong with that? Nothing. That's why instead of arguing against that (which is practically a fact by now), you should argue why it doesn't matter. Nintendo games are universally considered amazing, or at the very least pretty good, entry from entry. We can read between the lines and say that anyone who says that Nintendo sells games because their hardcore userbase gobbles up whatever they release is trying to downplay the success of these titles. But if these games are universally loved, does that really matter as a benchmark for how worthy of success a game is? No. And you almost said this in your OP, when you explain how games get as much success as they "earn", but you didn't quite get there.
What you shouldn't do and did is take that figurative point and address it to the most literal degree. People might argue that Nintendo fans gobble up anything from Nintendo. That doesn't mean they're being so literal as to not allow a few exceptions in there. In fact, you ironically highlight games which didn't sell as well as they should as "exceptions", but don't see how someone making the same argument could do the same thing. Especially since they aren't even "exceptions", they're universally hated or disliked games, which are obviously going to sell less. Bringing up the counter-point that Star Fox Zero didn't sell, therefore Nintendo fans don't buy "any first party title", is like saying Samus Returns didn't sell well so Metroid should die. There's a lot of circumstances that make it an irrelevant talking point, especially because I've never even read something as on the nose as the thread's title (definitely not in the literal sense at least).
Most of all though, I just find it lame because it comes off as so insecure. Two threads created and one thread devolved of Nintendo fans just being extremely defensive. Nothing personal and there's nothing wrong with that inherently, it's just eh. I guess I could be considered a Nintendo fan but, this kind of thing is just unnecessary. There's always going to be a few bad apples and you aren't going to change minds by making a thread for every possible misconception.