Oh and concerning what the OP says. If I were Bane I'd probably reply that those unable to confront and overcome the danger are weak and don't deserve the title of Sith. In that way the Rule of Two roots out the weak and incompetent, if both Sith were to die the Sith would eventually return again. But hey, it never actually happened did it so its not such a big danger after all. The reason why the Rule failed was because Vader abandoned it, really Sidious should have realised his apprentice was weak and broken and destroyed him - then found a new one. That is what Bane would have done.
Palpatine himself is interesting to discuss in light of the Rule of Two, because it's questionable how strongly he followed it. After all he took Darth Maul as an apprentice before he killed his own master and began his training in the ways of the Sith. Granted, Zannah did something similar to Set Harth, but she never named him a Darth or truly trained him. Also, didn't Sidious kill Plageious in his sleep? I'm not sure if Bane would consider that in the spirit of the Rule of Two. He said the title had to be "wrenched from the all powerful grasp of the Master." Of course you could argue that if Plaeguis was trusting enough to let Sidous kill him in his sleep then he was too weak to lead the Sith, but it's still debatable.