Just a thought, but maybe the reasoning behind the lack of National Dex is because there could be a lot of brand new Pokémon (Think RBY/BW). I mean, it might not be the case, but if it was, would that make the exclusion of older 'Mons more justified?

I'm still iffy about it, myself, but I tend to use all new Pokémon in every new Gen anyway, so it probably doesn't bother me as much as some other people.