Hollywood (assuming that's what OP is referring to) is BUILT on unions. Nearly every job on a film set is unionised. If the studios simply replace the actors and writers, good luck getting gaffers, camera people, electricians to work for your set. Drew Barrymore essentially has given herself a horrible name in the industry because she scabbed. Hopefully no one else will.
If the object of a strike is to obtain from the employer some economic concession such as higher wages, shorter hours, or better working conditions, the striking employees are called economic strikers. They retain their status as employees and cannot be discharged, but they can be replaced by their employer. Like someone else said, a strike and what happens during is a very complicated process and also varies by industry, for example, my CBA has a no-strike clause. It doesn't mean we wouldn't go wildcat at some point if pushed but it would take a lot. If we did go on strike, would the state come in and replace all the professors? Highly unlikely but they could try. The last teacher strike in Mass. was a wildcat that lasted 2 days before management gave in. So the tactic can work.