Wait until you see the regional dex. X/Y had over 450 Poke'mon in the game (Not counting Megas). There's a lot of Poke'mon that I'm sure nobody will miss like Zigazoon, a lot of UU Poke's, and Legendaries. Then there's all the Pokemon that fill the same role as Poke'mon from previous gens. Like Conkledurr and Machamp.
Also, there's no reason why they can't slowly patch the rest in over a year.
Actually, there is a reason they can't do that. It's called Game Freak. They never patched their games for parity before... OR/AS introduce a couple new features and forms, oh well I guess X/Y has to be incompatible. US/UM introduces a few new Pokémon, oh well S/M is now incompatible. They don't do patching because they want you to spend money on their half-assed games.
Also, it doesn't matter whether the Pokémon that are missing are memorable or not. First, because, as this thread has already shown, every single Pokémon has those who care about it. And second, because completing the National Pokédex is a big deal for Pokémon players, it's always been the main thing about the franchise, and that's getting removed for no good reason other than laziness. It's one thing to require players to bring Pokémon over from different games to complete the Pokédex, it's another to have those Pokémon missing entirely.