Every person?... more like 3 people. Dog being Sabretooth is far-fetched considering the care of attention Wolverine's origin was written under. Did Origin seem like a far-fetched story? No, it was pretty solid and rational. There was a lot of thought put into it.
Also, the most obvious evidence is Dog has dark brown hair, almost black hair. Sabretooth is a blonde. Please don't tell me his mutation makes him blonde.
I really don't think that Dog being Sabertooth would be far fetched at all. It fits easily really.
For one, Sabertooth has always seemed to hate Logan intensely, more then any other of Wolverine's villains. He obviously has some sort of deep seeded hatred for him. It would make a lot of sense if that dates back to the relationship dog had with young Wolverine. Dog was jealous of what James had. James had a father who loved him, a mother, he was rich, and most of all, James is always around the girl Dog had a crush on.
Second, Sabertooth has memories of an abusive father, but we obviously don't know how accurate those memories are because of Weapon X. Dog had an abusive father. Weapon X is known for taking memories and tweaking them a bit, so it's not too hard to see that they could have done the same thing there.
Thirdly, Sabertooth has made it a point to try and destroy almost anything good in Wolverine's life, especially his girlfriends. This would make sense if he was still harboring deep seeded feelings of jealousy because Wolverine had all the things he didn't as a boy. Not only that, but Wolverine had the attention of the girl Sabertooth (if he was Dog) had a crush on, so Sabertooth tries to take away every girl Wolverine ever has.
And the hair color actually supports my theory.
That doesn't look like black hair to me. In fact, it looks very similar to the dirty blond hair Sabertooth sports.