You have your logic a little mixed up there. DQH is PS4 only in the West because the PS4 is selling well in the West. It's selling not nearly as well in Japan (no home console is), so it's likely that, at least in Japan, it will be on both PS3 and PS4 to make up for sales they wouldn't get from it being a PS4 exclusive. It doesn't push more PS4 systems in Japan that way, which is certainly problematic in the long run, but it sells more copies of the game to the market where it will sell the best by far to begin with.
This isn't getting released soon though.. the PS3's situation in Japan will have caught up with its western situation by then