Can Splatoon sell more than FFXV in Japan?

[Miyamotoo]

If we talking about "Can Splatoon sell more than FFXV in Japan" we need to consider few facts:

-Like OP mentione FF has been on a bit of a decline in Japan in terms of sales. Second, PS4 is selling bad in Japan in comparison with other PS consoles and very likly will be worst sold PS console in Japan, so FFXV probably will have lowest instal base than any previous FF game in Japan. So I don't think that FFXV can reach 2m in Japan on PS4.
-Splatoon at end of this year will be around 1.3m, from moment it was launch back in May it always been in top 10 sold games and very often in top 5 sold games in Japan, game become some kind of cultural phenomenon, saying that Splatoon LT numbers in Japan can reach around 2m.

But there is one more factor, FFXV want be PS4 exclusive, there is chances that game will release on NX too, in that case FFXV definitely will sell more than Splatoon, but I think Splatoon will sell more than FFXV on PS4 alone.