I agree with those that have said that they doubt that the Pony line would be doing any differently in the absence of EqG. I mean, both toy lines sort of serve their own niche, and EqG seems to be Hasbro's attempt to ride the Monster High trend while tying it into the already successful Pony line. And honestly, the EqG line has to be doing well enough, otherwise there likely *wouldn't* be so much more attention put on it.
Me, personally, I really don't like dolls. At all. I have bought ONE EqG doll, and that was just to get the Rarity brushable that came with her. I still have the doll herself, still in her package, and don't know what to do with her because I don't collect them and can't justify tossing her, since she's perfectly nice. THAT is the reason why *I* don't like EqG dolls. Dolls just aren't my cup of tea.
Hasbro, as a business, is going to follow the money. They are going to keep releasing the Mane Six + a few as long as they continue to sell. And there are going to be collectors like me that are torn between the "gotta catch em all" mentality as far as variants go and hoping to put pressure on them by abstaining from purchasing. I do feel, though, that if their profits for the ponies themselves drop, they might just discontinue the ponies instead of doing what we want and adding variety though. On the same note, if Pony popularity drops too much, but EqG popularity stays up, I do see the dolls outliving the ponies.