Then that function is close to useless to me. Too bad :/
And this is generally going to be true of most players. Going further, what percentage of their customer base would have access to two 3DS systems, just to be able to evolve their gravelers and kadabras, or to get both a mankey and a meowth? I know that this is what people who played the originals had to go through, I was one of them, but through the games from Gen IV and onward, Game Freak has bred a culture of expectations that doesn't have to involve borrowing your brother's Game Boy, or requiring you to have direct access to two compatible games.
Restricting the communications of these games to local wireless only would not only contradict their philosophy of encouraging player interactions, but it would also put off many of the newer players who actually do expect to be able to battle and trade their pokemon on a global basis, as well as several older players who have become accustomed to the current easily interactive culture.
In addition to this, they already have the capability to add online interactions, and they've had this technology for the past decade. It would be more damaging for the sales and reputation of the games in the long run, I think, to not implement the very basics of communication to the Cable Club, something that they already have to change, than it would be to leave the games as isolated as they were twenty years ago,
only for the sake of nostalgia.
Overall, the games will see much better sales if they actually give a reason to get the games beyond simple nostalgia. Far more people will buy the games if there is an actual reason to buy them, and for most pokemon games, much of the purpose of playing comes through player interactions. Isolating these games would, for most of the people who do buy them, simply give the end goal of catching Mewtwo and getting that "Uber Special Level 100 Charazard", before they would eventually be left to gather dust. This isn't a smart business plan for games that were openly designed with the concept of player interaction in mind, and when the avenues for those interactions are wider than ever.