No one thought of Sulu as a main character during the entire run of the show because he wasn't one, Takei was a day player and Sulu a minor and easily replaceable character. TOS has a main cast consisting of three actors, Shatner, Nimoy and Kelley.
Exactly this. I recall he disappeared to do other shows for half-season or so and production didn't even flinch. You could not have repeated the same trick with Shatner, Nimoy or Kelley.
He was to TOS what Colm Meaney was to the first few Seasons of TNG.