If young was the way to go then we could have kept David alive. Not the greatest of actors but still......
Actually, I thought Merritt Buttrick was a fine actor.
And it was intended to keep him for whatever future the franchise would have (ie. telemovies?). Until Nimoy indicated he wouldn't mind returning to the ST franchise, Harve Bennett had no intention of killing off his rebellious "young Kirk" character, who seemed well-paired with "Young Vulcan" character, Saavik. As Bennett told a "Starlog" interviewer, Kirk was breaking the cosmic balance by getting Spock back from the dead, and he had to forfeit even more to put things back into balance. Thus, he gains Spock, against all odds, but must lose David and the Enterprise.
Bennett also realised that the only way to save Carol Marcus (for future possibilities) was not to involve her in ST III. Had she also been in the film, she would have been forever tarnished by David's protomatter cheat, and she'd have had to die too. Bibi Besch had been concerned that Bennett had not liked her work in ST II after all, but that's how he explained it to her.