I was alive AND an adult when TWOK came out and I can tell you that not everyone thought TWOK was "it" for Star Trek. People who have a grasp how movies are made, even then, understood that if a film is successful enough it will often spawn a sequel. Sure, Paramount may not have had plans for another film after TWOK while it was being made, but that's typical of Hollywood: "let's see how it does and if it goes like gangbusters, let's talk about making another."
As to the actor's perceptions of if it would be the last film, that's their perception. It doesn't reflect what was going on in Paramount's management, which most of them probably had little to no interaction will.
As to the actor's perceptions of if it would be the last film, that's their perception. It doesn't reflect what was going on in Paramount's management, which most of them probably had little to no interaction will.