Well the two reasons you mentioned would be rectified by a re-release of RSE, although that's just my hunch. In fact this generation hasn't even had a Safari Zone-type area yet; perhaps those Pokémon not available in both regions will be available there.
And really, the amount of people who play pokemon and don't have internet access is fantastically minute.
Though I still dream of a day they bring back the distribution methods of Gen 1. The ability to catch all the Pokemon, except version exclusives, within the game was very nice. Even if they made it accessible through limiting Pokemon throughout the time of day/season/area/outbreak, it would still be nice to have back.
As you said yourself, though, Pokemon games within the same generation do share some families. A new game wouldn't consist of nothing but what they still need to make available, obviously.
Besides, IIRC, the only 'mons that weren't available in DP via some means were some starters, legends, Tropius, and Tangela/growth. That didn't stop GameFreak from releasing Platinum or HGSS.
It's kind of inevitable isn't it? The population of Pokemon increases by a little over a hundred with every generation, yet the geographical scale of the game remains fairly small. I mean, we do get larger looking cities with the improvement in graphics, but the game hasn't really have much diversity for Pokemon locations. I was rather annoyed by the way BW1 try to fit tons of new Pokemon in that couple of grass patch in the post-game area. It's just unrealistic in terms of game design. Even if the two BW2 games had each taken up approximately half of the total Pokemon species such that we have a hundred or so version exclusive, the game is still lacking enough prominent locations to place the Pokemon in fitting location and form some sort of ecological system.
I've actually never seriously try to "catch 'em all" starting from the second generation, since I often restart my game a couple of times before I settle down with one playthrough, after which I'll only breed and train a couple of Pokemon I like.
I think they have a good amount of areas available to catch Pokemon, I just think they need to put more Pokemon in each area, which is very doable. They could just implement a more vast encounter % system and make each area very fruitful w/ Pokemon. Especially post-game where they could unlock more Pokemon per-area after you have beaten the game.
I wasn't saying the point was to cram as many Pokemon as they can, I was saying that they could have added a lot more than they did. And they definitely have the means to do so