Well Granblue had an estimated 22 million players worldwide last monthIs there enough fans of these games in Japan who aren't Smash fans that would buy in just for those characters? I'm not sure there's enough to justify it.
DQ still isn't owned by Nintendo so more sales isn't a gain for them. The only possible gain I can see is if they expect making the series popular could sell more systems somehow.
I'm pretty sure not all of them are going to buy Smash.
DQ is an entirely different beast.