So GF said they are working on backwards compatibility. I've heard mixed reactions to this statement, while some believe that it is great that GF is trying to make Gen 5/4 games connect with Gen 6, others believe that they are just saying that to save face.

GF said they were having trouble 'making the DS and 3DS connect'. Some people believe this to be rubbish while others believe it's more complicated because of the new type, which complicates Hidden Power and the way it interacts with IV's.

Now with all that aside, what would happen if GF failed to include BC?

I think that although it is possible to include all 'mons in XY it would be extremely difficult and unrealistic.
So, if we assume that they won't include all Pokemons in XY imagine how the metagame could be affected if some 'mons were to become unavailable. Furthermore, a whole bunch of fans would be upset that their favorite Pokemons are not included.

Some people have suggested that with no BC Gf would be forced to make Hoenn remakes but it would be a difficult process to turn that region into the new 3D world that is Kalos. If they did do Gen 3 remakes then there would still be 4 gens of unavailable Pokemon.

But even if they did a 50/50 split of Pokemons in XY and Hoenn remakes it would be at least a 2 years before we saw the Hoenn remakes.

Tl;dr How would the lack of BC affect Pokemon's metagame, fanbase, and future games/remakes in relation to the available Pokemon in X/Y?
Keep in mind that this discussion is unrelated to your feelings about transfering your IV bred and EV trained Pokemons.

Pic semi-related I guess