Why do people think DQIX will sell monumentally better than VIII?

[okr]

DQIV DS sold 1.26 million in Japan, even DQM: Joker sold 1.5 million.

Zelda: PH sold 0.93 million (I mention Zelda here as the series is not that huge in Japan and the PH sales were quite a surprise).

If even spinoffs and remakes of DQ and a new Zelda DS sell one million or more in Japan alone, imagine what the next regular entry of the 2nd biggest Japanese franchise (behind Pokémon) can do.