What? It worked once, and it gave them a win, but it won't work again.
The head was removed with the death of Sidious, and the redemption of Vader. Even had Vader not been redeemed, both he and Palpatine would have been blown up with the DS2. Either way, the sith lost yet again.
You're absolutely right. The Sith did lose. But my point is that they were able to do so much more than anyother incarnation of the Sith Empire. And that was because of the Rule of Two. Not to mention Sidious and Plagueis' scheming skills.