However, the way I've read it is that it was a combination of factors. First, Meyer wanted the Alley version of Saavik, not the Curtis version, and Alley either couldn't or wouldn't take the part. Second, Catrall, who was Meyer's second choice to fill the role in TUC, didn't want to play another Saavik. And, third, they started to question whether Saavik should turn traitor. All those things together contributed to the decision.
I would imagine this to be the reality.
Sounds plausible to me. It's usually a mistake to assume that artistic decisions are made for one reason and one reason only, especially when you're dealing with something as collaborative as a major motion picture based on a long-running series. People consider several factors, add up the pros and cons, and then make the decision . . . .