They could just let us catch all the Pokémon in Gen VI instead of making us rely on games that came out years ago. The start of Gen IV was quite annoying with the reliance on Gen III games. Gen V never actually fixed that problem - Gen IV is still the only permanent source of several Pokémon legendaries and the like.
Yeah, but there's still the fact that people get attached to their Pokemon. I myself have Pokemon in my Black 2 that date back nearly 10 years, and I would hate to have to leave them behind.
You can have your nostalgia as long as it doesn't make the new games worse. I'm happy with not being able to use my gen I/II Pokémon if it means better mechanics and all gen I/II legendaries and starters being obtainable in newer games.