I think it's both. FF15 is the highest selling game because it's the highest profile release there's been (until DQ, it will likely remain so, and DQ may not even pass it thanks to the 3DS and Switch versions) -- but as a series if you plot a year-on-year or game-on-game decline the decline FF15 has had is higher than the average and higher than the overall decline in the Japanese market between 2009 and 2016. Like, if the entire industry is down, okay, we expect all games to be down, broadly, except for wildly successful exceptions (such as Persona 5). So, then we plot: what's the average percentage THE INDUSTRY is down? If a game is more down than the average, there is clearly a factor besides the decline at play. This is the case for FF15 in Japan. Surely this makes sense to you? As far as the reason beyond the wider market, I think it's mostly about the Western influence exerted over the game's style being rejected by a segment of the player-base, and quality/polish issues, which Japanese games surely rag on more. It's not exactly a massive mystery.