Patrick Stewart was paid $14 million for ST Nemesis. Now that's a tragedy. Based on the commercial success of Transformers $18 million for a lead would be dirt cheap.
Take away Patrick Stewart from TNG films and no one would see them.
When Star Trek: The Next Generation
debuted many thought that the fans wouldn't watch a series with Star Trek
in the title without the original actors on board. Twenty-five seasons of television without them later, that's obviously turned out to be false.
Now, Paramount certainly thought Stewart was integral to the series. They gave him script approval of at least one film and significant pay raises from what he was earning as a television star.
But the only way to know if fans would skip a Star Trek: The Next Generation
film without Patrick Stewart would be to release one; obviously, that's not going to happen. You're speaking definitively about something that's entirely hypothetical.
I will say one thing, though: if the Bourne series can be successful without Matt Damon (The Bourne Legacy
had worldwide grosses on par with all but the third film in the series), anything is possible.