What Sith goal justifies the Rule of Two? And was Palpatine fulfilling it or betraying it?

Question

The Sith adhere to the Rule of Two: one master and one apprentice. The apprentice is taught that once they surpass their master, they must take his (her?) place by killing them. In this way, the Sith order increases in power through a Darwinian-like process of attrition.

This means that the Sith cultivate an extreme (if somewhat twisted) form of altruism. The master willingly sacrifices his own life by training an apprentice who will ultimately kill them. All for the greater good (evil?) of the Sith order.

To willingly submit himself to such a harsh requirement, there must be some goal that the Sith master values higher than their own life. It can't simply be the desire for power or domination. If you were that sort of person you would want to enjoy your power, ruling the galaxy etc. You wouldn't train an apprentice who would take it all from you at the first opportunity.

So the Sith must have some higher goal than that. What is the goal that they are devoted to? And was Palpatine on his way towards fulfilling that? If Palpatine were successful in becoming immortal then there would be no more Sith after him. The Rule of Two would cease to operate: nobody could kill him, hence there could be no future Sith Lord more powerful than him.

Does this mean that Palpatine expected the Sith order to culminate in his own person, or was he opting out?

(Yes I know this is really two questions but I believe they are related).

Answer 1

Legends answer.

The objective is for the Sith to survive. Darth Bane recognized that the disciples of the Dark Side will always want more:

"I can't accept what Lord Kaan preaches," he confessed. "He says we are all equals, but if all are equal, then none can be strong."

...

"Do not believe everything Kaan says," she warned, and he could hear the naked ambition in her voice. One to embody the power, the other to crave it. "Once the Jedi are destroyed, many of his followers will discover that some of us are more equal than others."

From Darth Bane: Path of Destruction

So, Darth Bane was aware that the Dark Side would pit its disciples one again another as they were craving for more power. The problem there was: a lot of weaker Sith would work together to kill a strong one and with that, they'd weaken the Siths.

He was absolutely convinced of this and with that, he indoctrinated his apprentice, who in turn then again indoctrinated their apprentice with the thought and so on. Plus, having an apprentice gives you power up to the point where you are betrayed. So it is a normal thing to have an apprentice.

Palpatine was not the first to crave for immortality. Darth Plagueis was Sidious master and he already craved immortality. And with becoming nearly immortal, he wouldn't betray the Rule of Two. The objective of the rule is for the Sith to become stronger. If Palpatine had become immortal, always learning more about the Force and becoming more powerful, he'd have fulfilled the ultimate objective of the rule of two: making sure that the Sith survive and grow more powerful.