True the network eventually wanted more Spock, but not initially. When the series was still in the pilot stage Roddenberry had to fight the network to keep Spock in the series. Eventually the network gave in to Roddenberry, under the condition that he "keep the guy with the ears in the background", a demand which Roddenberry mostly ignored. So, even if Roddenberry had wanted Spock as the main character (and I'm not implying that he did) there's no way the network was going to allow it.
I know that perfectly well. My point is that it was fan reaction that made Spock the breakout character. Once the network saw how hugely popular Spock was with the fans, then of course they dropped their resistance.
Just because Roddenberry insisted on keeping
the character, that doesn't mean that he was always meant to be the central
character. It just means that Roddenberry didn't want to give up on having an alien character in his show set in outer space. He wanted that alien to be an important part of the show, but not the core of the show.
"By November 4, the first two actors were set for the pilot as Gene sent the following memo: Please make a deal on Magel (sic) Barrett to play "Number One" and Leonard Nimoy to play "Mister Spock". . .The role of "Number One" . . . had been written for Majel. . . no other actresses were considered for the part."-Star Trek Creator by David Alexander, pg. 210
Said memo was issued after
the pitch document I mentioned before, which is dated March 11, 1964 and already includes the entire cast I listed above. Therefore, it does not reveal anything whatsoever about the order in which the characters were created. I think everyone knows that Number One was written for Majel, but that tells us nothing about when
the character was created relative to the others, or when Spock was created.
I realize Spock was not initially intended to be the main character of Star Trek, it just seems like his importance to the series grew to the point where he became the main character.
That's a better way of putting it. As I said, that's certainly what the network and the fans wanted. But there was always an effort made (by the producers and definitely by Shatner) to ensure that Kirk remained on a par with Spock. I'd say Spock became more "the de facto star" than "the main character" per se.