While it wasn't totally unfeasible for TNG to make more than 4 movies.
Except, the pattern of rising salaries for Stewart and Spiner was making the budget trickier with each film. Or Paramount had to have been brave enough to proceed without one or both of them.
TMP commenced as "Phase II" without
Nimoy. Saavik and David were groomed to take over from Nimoy and Shatner in future Spockless/Kirkless ST followups to ST II. They weren't needed
, but they were on hand. Each movie was a reunion, after the initial absence of the characters for ten years.
The success of TNG films, after seven years of the characters together in ensemble, hinged on the ensemble effect, even though two cast members insisted on maximum participation/salary, but TNG movies' success still hinged on the presence of the ensemble.