Well, it's a matter of personal preference, but the pilot doesn't have to explain how anyone got anywhere. All it has to do is get the audience all they need to know to follow the story and series premise. It only matter that they are there. We don't know how Kirk got command of the Enterprise, how and why Uhura and McCoy came on after WNMHGB, and so on. The series being patterned after the Navy, they simply were assigned and/or worked their way up the ranks. For me, it never mattered. And since the episodes were shown out of order, in fact they were expected to be shown out of order, there was never really a progression. Sulu became a helmsman because that's what Roddenberry wanted him to be, going forward. It put Takei in a much more prominent light and with a science officer on the bridge played by the second lead, a ship's physicist becomes redundant. You notice, after Mitchell died, Sulu wasn't at the helm, it was Scotty. Besides, wouldn't there be someone better suited to take the helm than a scientist?
YMMV
Actually, Scotty was at the Navigator's station not the Helm, from this picture someone in a blue tunic is at the Helm station, perhaps Communication's Officer Alden.
The picture of that scene is provided by www.trekcore.com.
Navigator NCC-2120, USS Entente
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Helm was on the right for this pilot. Scotty is sitting at the helm, which was Mitchell's station.