The whole basis of the rivalry was that the Red Sox couldn't beat the Yankees. They did.
And it's still a rivalry. Just a basic one.
Well, then it wasn't ever a rivalry.
If one team comes out on top all of the time, then there is no basis for the rivalry.
When I played soccer in high school, our biggest rival was a team that we beat, then they beat us. You can't have a rival against a team that always beats you. It isn't a true rivalry then.