Wouldn't Spock be the oldest TREK character? He's the only hold-over from the original pilot.
"We like the pointy-eared guy, scrap everything else."
Actually NBC hated "the pointy-eared guy." They were afraid he came off as too satanic and strange. But Roddenberry fought to keep him. They also wanted to keep Jeffrey Hunter as the captain, but Hunter himself (or his wife) decided that wasn't going to happen. As for Number One, the network would've been happy to keep the character if she'd been recast -- they didn't like the nepotism and potential scandal of the producer casting his mistress in the role. But rather than admitting that, Roddenberry dropped the character altogether and found a different, less central role for Majel Barrett.