And I'm not arguing this was ever mapped out in any definitive way. It probably mostly depended on how charismatic the new actors were, and how well their characters went over with the audience.
And how much Paramount wanted to pay the aging Original Series stars for a ongoing movie (or TV) series.
I would've loved seeing Captain Saavik and her crew in a weekly TV series. I think Alley could've headlined such a series quite well . Merritt Butrick didn't really have Leading Man charisma, but he could've been effective as a second or third banana.
In any event, Nimoy's desire to come back as Spock pretty much blew any advance planning out of the water. Which is fine; we got a few great movies out of the deal.