When did this retconning of the rationale behind the Rule of Two happen?

Question

There are two rationales for the Rule of Two at different points in the past. When did this retconning happen, and why?

The initial rationale for the Rule of Two is:

• The strength of the Force concentrated in a Force user depends on how many practitioners there are at any given moment. The more of them there are, the more the Force is diluted among them, rendering each individual weaker in the Force on average.
• The Sith therefore maintain the lowest number possible to concentrate as much of the dark side in each Sith Lord as possible without endangering their order's continuity too much. Meanwhile, the Jedi grows in number, weakening the concentration of the light in each individual member as a result.
• As the Sith further their mastery of the dark side, their strength grows further, while the Jedi's budding complacency and arrogance develops, causing their strength to wane further. Eventually, the Sith's power in the dark side will eclipse the Jedi's power in the light, and that will be the moment to strike.

This philosophy was still in place when Episode I came out. When the book Darth Plagueis was published, it described Sidious - at the moment of Plagueis' death by Sidious' hands - feeling a huge shift in the Force as, even for just a moment, the entirety of the dark side is concentrated within just one Sith Lord.

At some point later, though, it had been retconned to the current philosophy that persists into Disney canon. For one, an individual's strength in the Force is no longer dependent on the number of practitioners. The reason Darth Bane adopted the Rule of Two has been changed to such:

• Sith crave power, authority and supremacy by nature. Infighting is inevitable.
• However, when the Sith are many in number, the weak can and will form alliances to overpower the strong - it is their only way to live. This process will repeat, however, until only one is standing, and it will not be the strongest.
• The master of each new generation will come from among the weak of the last, and left to persist, the Sith will inevitably doom themselves to extermination.
• The solution is to restrict the order to only two, so that the apprentice can only defeat the master when he/she has indeed become the stronger. That way, the master of each new generation is stronger than the last, and the order will therefore benefit, grow and strengthen from this infighting instead, eventually growing strong enough to destroy the Jedi, who are likewise succumbing to complacency and arrogance.

The benefit of the Rule of Two to maintaining the secrecy of their existence from the Jedi's awareness is the same in both versions.

Edit: Many asked where I found the first. It was many years ago, but I can find the same being mentioned in the Sith Wookieepedia article:

Learning from the order's past mistakes, Darth Bane established The Rule of Two and restructured the Sith so that there could only exist two members at a time: a master and an apprentice. This would concentrate the Dark Side of the Force into two powerful beings rather than spread it amongst legions of ineffective warriors.

Answer 1

I think your premise is wrong, there wasn't really a point where "concentrating power" was the main rationale for the Rule of Two--the rule seems to have been invented for The Phantom Menace which came out in 1999 (it didn't feature in any EU fiction before that), and if you look at page 4 of this interview Bill Moyers did of George Lucas from April 1999, Lucas says that you could only have two because of infighting:

One of the themes throughout the films is that the Sith lords, when they started out thousands of years ago, embraced the dark side. They were greedy and self-centered and they all wanted to take over, so they killed each other. Eventually, there was only one left, and that one took on an apprentice. And for thousands of years, the master would teach the apprentice, the master would die, the apprentice would then teach another apprentice, become the master, and so on. But there could never be any more than two of them, because if there were, they would try to get rid of the leader, which is exactly what Vader was trying to do, and that's exactly what the Emperor was trying to do. The Emperor was trying to get rid of Vader, and Vader was trying to get rid of the Emperor. And that is the antithesis of a symbiotic relationship, in which if you do that, you become cancer, and you eventually kill the host, and everything dies.

Also, reading over Plagueis' death scene in the novel Darth Plagueis (which is shown twice, both in the prologue and again in the final chapter), I don't see any clear indication that the Force suddenly became more concentrated in Sidious when Plagueis died, though there is this statement in the prologue:

Two beings in a galaxy of countless trillions, but what had transpired in the suite would affect the lives of all of them. Already the galaxy had been shaped by the birth of one, and henceforth would be reshaped by the death of the other. But had the change been felt and recognized elsewhere? Were his sworn enemies aware that the Force had shifted irrevocably? Would it be enough to rouse them from self-righteousness? He hoped not. For now the work of vengeance could begin in earnest.

I don't see this as a statement that the Force had become more concentrated in him though, I think the "shift" referred to here is just about a change which will have huge effects on the future history of the galaxy, a sort of change in destiny (or Sidious making the crucial act that would secure his destiny). If you have a copy, note that the two paragraphs immediately before this one are all about seeing the future and divining the "will of the dark side".

There's also this part a little later in the prologue:

The dark side had made him its property, and now he made the dark side his.

Breathless, not from exertion but from the sudden inspiration of power, he let go of the sill and allowed the monster to writhe through his body like an unbroken beast of range or prairie.

Had the Force ever been so strong in anyone?

Sidious had never learned how Plagueis's own Master had met his end. Had he died at Plagueis's hand? Had Plagueis, too, experienced a similar exultation on becoming a sole Sith Lord? Had the beast of the end time risen then to peek at the world it was to inhabit, knowing its release was imminent?

It's hard to tell how much of this "exultation" and sense of power was psychological, due to his sense of triumph and no serious opposition left to his plans for absolute power. And even if the Force had genuinely become stronger in him, it could be that this happens whenever an Apprentice becomes a Master, just because the dark side respects power, not because the dark side has a fixed total that's divided up between existing Sith (if it was then anytime a Master initiated a new Apprentice, the Master would feel a loss of power).

And in any case, even if the author was to some extent thinking in terms of the idea of the small numbers of the Sith allowing their power to become more concentrated, this doesn't seem to predate the infighting explanation for the Rule of Two, nor would it be inconsistent with the idea that infighting was the main reason the Rule had arisen in an in-universe sense.

Answer 2

The Jedi vs Sith comic series had the reason for making the rule of 2 as to stop the infighting prevalent among the Sith that was making them weaker. This was published in 2001.

Darth Plagueis was published in 2012, and has the differing reason that you gave.

The retcon appears to be the other way around to how you describe, with the strength of the Force being stronger a newer addition to the lore.

I have only ever heard of the 'stop infighting' reason as established in Jedi vs Sith.