True, but aside from said fossil Pokemon, the only way to get the rest of them that are available without the need of Pokebank would be through the Friend Safaris, and as we all know Friend Safaris requires one to register FCs. Can't get the Friend Safaris? Off to the GTS/Trade Corner you go.
I mean, sure Friend Safaris increase the amount of Pokemon encountered in the post-game, but hey, it all depends on the FCs you've collected. My Friend Safari was actually useless when I started to search for dex entries, since I had no one registered in my Friend List yet.
I forgot to mention that I'm merely speaking the availability of Pokemon in the Kalos dex alone. 451 is a whopping lot for a regional dex. If one has 2 3DS and a copy of both Pokemon X and Y, after including the post-game fossils and without counting the Friend Safaris, the maximum amount of Pokemon that one can get without using the Pokebank/GTS is 467, which is the 451 available at the Kalos dex plus the extra 8 lines of fossil Pokemon available post-game.
And since the Pokebank-only Pokemon have already been bred like bunnies since Pokebanks initial release, they're actually quite easy to find now.