Yes I know dragon Quest is big but I was just referring to games released this last generation since it's a more accurate representation of how games are selling today in Japan not how they used to sell. Final Fantasy 15 is a testament how once massive franchises don't guarantee massive sales anymore. And I do agree the upcoming dragon Quest will do big numbers and surpass FF15 I doubt it will reach the heights of the previous entry's at least not on a single platform.
On a single platform? I concur, without a doubt. On all three? It could match DQIX, after all, the combined user-base will be greater than the DS's 30 mil in 2009 by the end of this year. As well as that, double, or potentially even triple-dipping will be prevalent due to the vast differences in the versions of the game. You have to remember that FFXV is very special example. Final Fantasy has been affected by Westernisation and long-term brand-damage. Dragon Quest has suffered neither of these, so to expect a decline as severe is unreasonable.