Yeah some people will assume you're crazy for saying that haha
But I wouldn't go so far as to say that. Splatoon as shown to be game that has possessed fantastic legs since it launched over two months ago in Japan. To reiterate: I'm talking about Japan, the place where console games drop off the charts before you can even blink. Being able to hold it's ground so far (and into the foreseeable future) just shows that Nintendo has been able to create a game with staying power.
Now on to the main event. The fact that FF XV is (basically) exclusive to the PS4 means that it will have more of an uphill battle to get to the numbers of it's predecessors. But if it's one thing that it does have over Splatoon is that it's a recognizable IP. Couple that with the growing PS4 userbase in Japan and FF XV has enough room to sell pretty well. Though I wouldn't expect it to match Splatoon's LT Japan sales first week, or even first month, for that matter. But I think that FF XV will come out on top over time.
But really, if Splatoon ends up outselling FF XV in the end then I'm seriously going to laugh my ass off for an entire day or more.