That wouldn't really be logical. They'd get less revenue from that.
Plus, Diancie is in the game already. Also, Masuda is firmly against paid DLC for main Pokémon games.
Not necessarily, the way I see it making new editions of games was really the old fashioned version of DLC. And now as opposed to trying to convince people who already own the game to shill out for an almost identical copy of the game, or encouraging a group of folks to hold out on buying pokemon games all together until the the superior version is announced, they can simply market an upgrade DLC pack to absolutely everybody.
Did he say this about any form of DLC in general, or just the concept of barring catching certain pokemon behind a pay wall? Because I really do not see a difference between that the current release model on a fundamental level, other than less convenience. It can be the other way around, where Diancie is distributed in some way for free as a mystery gift, and then it leads into some extra quest storyline.