To ditch the rest of the ensemble beyond Stewart, Spiner, and Dorn would have angered a great deal of the fans. If that made big financial sense for the studio, I can see them trying to weather that storm. However, the truth is that it was the salaries of Stewart and Spiner that were causing the budgets of the films to explode.
While I'm sure someone like Marina Sirtis or Gates McFadden was not being paid chump change by any means, I seriously doubt that eliminating them from the cast would have made a major impact on the film's budget.
The only way actor salaries could have helped the budget is if they had eliminated Stewart or Spiner, and they weren't about to do that.