Back on point...
I can understand all this attempt at retconning (including Lucas himself, post 1983).
People are trying to come up with an "explanation" for Palpatine wanting to get rid
of Vader and replace him with a new apprentice.
In the context of OT however there is no "rule of two" (never mind any motivation
for continuing to follow it once all the Jedi are thought destroyed, post Episode III).
ESB makes it clear that once the Emperor knew Luke Skywalker existed and had the Force,
he saw him as a threat and wanted Luke DEAD. Vader talks him out of it (obviously, because
once Vader realized he had a son with the Force, he wanted to keep him for himself). But
Vader's "subterfuge" is because of his ties of blood, not because of some ongoing
"Sith civil war," in the context of the movies.
So what do we do? Have Palpatine believe that Vader is "weaker" than he should be,
and he's just sitting around waiting for someone to come along who is stronger, just
for being younger and with fewer replaced body parts? Give me a break. Plus, when you
think about it, even in the Prequels, Palpatine "recruites" Anakin when he already has a
mechanical hand (just like he "tempts" Luke when Luke has a replacement hand).
Dooku had no cybernetic parts. He was an old man, sure, but physical age doesn't seem
to matter much (if at all) to Force users in combat.
I think it's obvious from the movies that Palpatine has no intention of replacing Vader
until he sees Luke BEAT Vader in combat. At this point it's a matter of pushing Luke
over the edge (letting his anger take its toll into bloody revenge against Vader for threatening
his sister). Perhaps he sensed that Vader was plotting against him, but there's no real
evidence in the movie that this is the case, other than that Palpatine suspects Vader has
a lot of interest in Luke beyond what he's letting on. In the end, Vader is loyal to Palpatine
(and Palpatine knows it) until the final zapping sequence.
But I think however we understand this dynamic between the two, there's no need to
resort to an explanation that somehow Vader's "potential" is lowered because of his body
parts. If anything, his injuries should make him angrier, and therefore ENHANCE his Dark
In the real world, bionics are weaker than the real thing. In Star Wars, we're lead to
believe they're just as good, if not stronger. Sure, Lucas has a bit of an "anti-technology"
streak in him, but when it comes down to it, for him what beats superior technology is
a matter of smarts, luck, and "heart" (not how much meat you have on your bones,
or else Jabba would surely have beaten Luke & co).