that just seems to me to be the whole reason behind the rule. The Sith needed to survive, so 1 master 1 apprentice. Hard to detect in such a large galaxy. There is of course the question of whether there could be other sith out there, and that the rule of two refers specifically to pairings. No master having more than one apprentice, but that's another debate. Once the sith have plotted and manipulated long enough to wipe out the jedi, the rule of 2 becomes redundant. No need to hide anymore, they've won.