It's no reason to assume they won't either.The fact that FFG does well with their current product lineup is no reason to assume that they can turn anything into gold, no matter how bad a premise it may be to start off with.
With the star destroyer argument, there were many, concrete, definable reasons it would be too problematic. So far your entire argument against an additional faction has been "because they might screw it up".
I also note your just assuming its a bad premise to start off with. Basically you personally don't like the idea and that is so coloring your perceptions of it you can't see anything else. Vanor has been doing a much better job of arguing against the idea, in fact, he is the only one providing any reasonable arguments against it, and even his argument boiled down to "it doesn't add anything", which isn't quite true.
I've said before, I don't think it will happen. I don't think it is necessary. But I do think it would be fun and I think it could be done very very well without too much trouble on FFGs part.
When faced with the possibility of introducing radical change into a system which already works incredibly well, which is the most natural assumption? That the game will a) stay the same, b) get even better, or c) get worse?
Any of which I am ok with.