This. In the B2W2 special edition guide, there is an interview included, and I believe it is Masuda who specifically mentions the key feature and that the team did not have enough time to include it in BW.
As impossible as this sounds, I really do think the new features (with the exception of the keys) were made after the release of BW. Game Freak has been developing high quality games rather quickly lately, which is very impressive. So I don't think it's that BW were "left out," or even that it was a ploy for more money. It was just what happens to all game developers: crunch time got the better of them, and then they worked harder on their next release.