So he would have a crippled (by Scotty) ship in III, not be in IV or V at all and have the same scenes in VI, maybe. By then, people would have thought "Sulu? Sulu Who?" and a different captain would have been helming the Excelsior. If Shatner did this on purpose, he did Takei the actor a big favor.
Wasn't it Nimoy who wanted Spock dead in Trek II, or Harrison Ford wanting Han Solo to die in ROTJ? It's not like the idea of actors wanting their characters to arc, even if it means the end of paying work, is that unheard of. And look at Checkov. He got to "leave home" all the way back in Trek II, even though he wound up back on the bridge of the Enterprise again. The Captain Sulu TV series idea popped up very late, around the time he guested on Voyager, but I don't recall him saying that him getting the captain's chair was going to give him job security, just that it suited the development of the character. The lengths they went through to keep the senior officers from graduating to their own command or a desk job were getting kind of silly. Kirk had to find excuses to escape the Admiral's post twice and Spock was already captain's rank but not allowed to command the Enterprise, even after Kirk got demoted in Trek IV. It just didn't make sense unless you accept that the crew was indeed an ensemble/family and they all passed on promotions out of their social bond, which destroys the premise by haters here that it was never an ensemble and shouldn't be seen as one. So some people really want to argue both sides of this just to be contrarian. Like I said before, by the time of Trek IV, the cast was indeed handled as if it were an ensemble/family. That's how the fans saw it (the same fans who went to conventions in the early 80s and who grilled the cast endlessly about Shatner) and that's how people see it today in retrospect. So however TOS was conceived, that is what it evolved into, sometimes with good (Nuclear Wessels) and bad results (like Nichelle Nichols' striptease in The Voyage Home).