Honestly I think they made the right call getting rid of Rand. Whatever might have been intended with the character, the way she came across on screen was as this overly fragile and submissive servant that Kirk was a little too uncomfortably close to. And watching them together was just always really weird.
I come at it from the other perspective: they should have started writing her better. It's not as if they replaced her with a different character, they just stripped out the female lead and left the show to the men. Rand in the first draft of City on the Edge of Forever absolutely rocked. She accompanied the search party, escaped Beckworth's clutches, used her tricorder to overload the pirates' transporter, jury-rigged the damaged transporter, and took command of the security team while Kirk and Spock returned to the planet. Harlan Ellison was a friend of Grace and I suspect he did it as a f##k you Rand is awesome protest but what's more interesting is what happened to the replacement yeoman character in the rewrites.
The second draft featured a yeoman who, when faced with the errant crewman, ran off in fear to fetch the men. In the third draft, Uhura led the security team but had little to say except to tell Kirk she was afraid. The problem wasn't the concept of Rand, it was the writers' obsession with her as a victim who was repeatedly stalked, sexually assaulted, and harassed.
It's been reported that Rand put the writers in a really uncomfortable position. She was written to seem close to Kirk almost verging on being a romantic interest, but they still wanted Kirk to have random romances to make the series more interesting. When they wrote a scene with him having a romantic interest they felt like he was cheating on Rand. Not knowing how to balance this they simply removed Rand so that their hands could be untied when writing for Kirk.
I think this shows up the sexism of the times, that the female lead had to be written out so the male lead could romance guest characters rather than them both accepting that the relationship would never work so that Rand could romance guest characters too, instead of just being stalked and harassed by them.
I love Helen Noel as a character but with modern storytelling, they could have kept Rand as originally intended and Dr Adams' neutraliser could have totally gutted the attraction between her and Kirk, allowing them both to move towards a more professional relationship.
